In the race's history, only 8 riders that have won have gone on to win a Grand Tour. These riders are, in order from most recent, Nairo Quintana, Denis Menchov, Angel Casero, Laurent Fignon, Miguel Indurain, Greg Lemond, Joop Zoetemelk and Felice Gimondi. From 1981 to 1992, the race was open to professionals so that is why Fignon won it in 1988, way after he won his two Tour wins. Marc Madiot and Johan Bruyneel won the race during that time when it was called the Tour of the European Community. Lemond won his edition of this race by a massive 10 minutes (on Robert Millar and Lucho Herrera!) in 1982 and while he was already a full pro with Renault-Elf, this was the place to beat down everyone. So obviously some riders are able to transition their Tour de l'Avenir rides into at least one Grand Tour win. The race did go to a U26 platform after 1992 and then to all U23 in 2007.
Let's go to some of the bigger disappointments. Fedor Den Hertog was the strongest amateur of his time and was able to win with relative ease. This included an Olympic Gold Medal in the Team Time Trial as well as overall wins in the Milk Race, Olympia's Tour and the Tour de l'Avenir. Den Hertog was a perpetual amateur who refused to go pro for the longest time because he wanted to be a leader and have the freedom of choosing his calendar. The year that he won l'Avenir, the course was not nearly as mountainous as this year's edition and was filled more with small hills spread across 10 stages. Den Hertog won the race by 19 seconds on successful amateur Ivan Schmid, who would go on to win a Tour de Suisse stage. The rest of his pro career? A shell of his amateur career because he was so viciously marked by riders that he was never able to get away and while he went on to win a Tour and Giro stage, it was nothing what was predicted for him. He died in 2011 of prostate cancer.
Other riders who never hit their expectations from this race include Evgeny Petrov, who won the overall in 2002 after an incredibly U23 career but then never matched those successes; Sylvain Calzati won the 2004 edition ahead of Thomas Lofkvist and Christopher Le Mevel...well that podium just never reached expectations. The later two faired better and Lofkvist even won a few big races but was always hyped-up much more than he ended up producing. This critique isn't just reserved for GC men. Guido Van Calster won 4 stages in 1977 but never really panned out as a pro. Sebastian Chavanel won 5 stages over the course of 2003-04 in l'Avenir but he has been known better for being Sylvain's brother.
There is the "what could have been?" category. In 1978 and 1979, Sergei Soukhoruchenkov came to the race with his Soviet teammates and threw down the gauntlet. They were in their prime and the Europeans, who hadn't had too much contact with them, got their asses handed to them. In 1978, Soukho and the Soviets won 9 stages and swept the first 4 places and he has a 3'30" gap to 2nd place. In '79, he won by an even bigger gap. After winning the Olympic Gold Medal in the RR in the 1980 Olympics in Moscow, Soukho was denied a 3-peat in l'Avenir by upstart Alfonso Flores and the Colombians, who were making their first appearance in l'Avenir. Communists are among the biggest what could have been contenders because they never really got to race a full European schedule against all of the big teams until after the Wall fell. Olaf Ludwig was already 30 by the time he was able to join Panasonic. Before that, he had won 5 l'Avenir stages and the overall (among a shit ton of other things) but if only he was able to turn pro at 25.
There were others that used this race to make people say, "Oh he is going to be a kick-ass professional." Erik Zabel won 4 stages in 1994. He would win Paris-Tours later that year and then 6 straight Tour de France green jerseys starting in 1996. Edvald Boasson Hagen won 3 stages in 2006 as just a first year U23. Miguel Indurain won two stages in 1985, including a time trial, and then won 3 stages on his way to the overall win in 1986.
I could go on for weeks analyzing every Tour de l'Avenir since 1961 but that isn't the point. Just because a rider wins this race in spectacular or dominating fashion does not make them a champion in the professional peloton. This race might be the Tour of the Future but more likely than not, the guy that wins this race will not go on to major glory. Granted, that is including a whole lot of years where this race barely included mountains and in more recent years, specialized riders that have at least won a stage here have gone on to great things. For example, every rider in the 2009 edition that won a stage is on a professional team now and 9 out of the 10 from the top 10 are in the pro ranks right now. In any event, use the Tour de l'Avenir rider as a guide but for the love of everything that is sacred with cycling, do not dump the expectations onto them the moment after they win the race...because that is how you get a Romain Sicard.
EDIT: So let's talk numbers. To get a better impact on how those that are successful in this race go on to do in the pro ranks, let's examine the results from the Tour de l'Avenir from 2007-2010. The dates might seem arbitrary but 2007 was the first year (in recent memory) the race was full U23 and then I chose to stop at 2010 because that is the last year where no current U23s could have possibly races. Also, I'm choosing to define a professional as someone who is riding on the Pro Continental level or higher. That isn't to slam continental riders because a lot of the most badass riders ride on the continental circuit. I'm simply chosing that level because, for the most part, they make a livable wage and they also ride some of the biggest races as a pro. You might ride the Tour of California on Jelly Belly but that is a one-off and the next week you could be back to riding Tulsa Tough. Moving on...
The 2007 race was won by Bauke Mollema, who at that time was a cut above the rest. He beat Tony Martin by 44 seconds. The rest of the top 10 follows as: Andre Steensen, Simon Spilak, Benat Intxausti, Guillaume Levarlet, Craig Lewis, Kristoff Vandewalle, Frederik Wilmann and Dario Cataldo. 7 out of 10, that's 70% for those humanities majors, are still racing in the professional ranks right now. All of them but Craig Lewis are racing currently with Steensen (CULT) and Wilmann (Ringerike) on continental teams. All of them have raced on the professional level at one time.
The stage winners for the same year include Dario Cataldo (won 2 stages), Stephane Pouhlies, Kristof Vandewalle, Martin Kohler, Jose Herrada, Ivan Rovny, Rafaa Chtioui, Nicholas Hartmann and Oleg Opryshko. Out of the 9 stage winners, 6 are still active as professionals right now, 7 out of the 9 are currently racing and 8 out of the 9 were on the professional level at some time. The only outlier was Opryshko, who bounced around Ukraine and Eastern Europe for a bit. Hartmann is now retired while Chtioui raced for Acqua e Sapone and Europcar and is now on SkyDive Dubai.
2008: The top 10 on GC consisted of winner Jan Bakelants, Rui Costa, Arnold Jeannesson, Jerome Coppel, Andrey Amador, Marcel Wyss, Jarlinson Pantano, Tejay van Garderen, Damiano Caruso and Peter Stetina. All of them are professionals and 7 of them are on World Tour teams.
The stage winners from that year include Amador, Dmitriy Kosyakov, Coen Vermeltfoort, Dominic Klemme (x2), Ricardo van der Velde, Bakelants, Rein Taaramae, Arnold Jeannesson and van Garderen. Out of the 9 stage winners: 6 are current professionals, 8 out of the 9 are still racing and 8 out of the 9 were on the professional level at some point. Ricardo van der Velde got a pro contract with Garmin but fizzled out fast, bouncing around until he retired after last year. Vermeltfoort started out as a pro with Rabobank but after a tumultuous 2 seasons, he went back to De Rijke-Shanks and got his mojo back. Kosyakov is the only one never to be a professional, at least by my definition, and has ridden with Itera-Katusha for the last half decade.
2009: Top 10 GC was Romain Sicard, van Garderen, Sergej Fuchs, Jaco Venter, Michel Kreder, Daniel Teklehaimanot, Stetina, Nicolas Schnyder, Andrei Krasilnikau and Rafael Valls. This is one of the weaker years as 70% of the riders are pros (or rode professional) while 2 are not racing anymore in Fuchs and Schnyder (doping) while Krasilnikau is an amateur with the AVC Aix en Provence team in France.
Stage winners include Julien Berard, Jean-Lou Paiani, Marko Kump, Troels Vinther, Jonathan Castroviejo, Timofey Kritskiy, Andreas Stauff, Romain Sicard and Dennis van Winden. 7 of them are professionals now (77%) while all of them have raced on the professional level and are all still racing. Vinther (CULT) and Paiani (Roubaix Lille Metropole) still ride on a high level on the continental circuit.
2010: Top 10 GC was Nairo Quintana, Andrew Talansky, Pantano, Tom-Jelte Slagter, Mikel Landa, Romain Bardet, Thomas Bonnin, Michael Matthews, Darwin Atapuma and Wilco Kelderman. 9 are professionals with Bonnin being the only one not racing still. He did ride for Skil-Shimano so all of them have been pros.
Stage wins went to Taylor Phinney, John Degenkolb (x2), Anthony Delaplace, Yannick Eijssen, Romain Hardy and Quintana (x2). All are pros currently.
So let us tabulate these arbitrary numbers.
Of the racers that placed top 10 in the l'Avenir GC from 2007-2010...
-37 different riders placed in the top 10
-81% of the riders are currently racing professionally
-91% of the riders rode for as a professional (again, Pro Conti and above) at some point
-89% of the riders are still racing at some level.
-4 riders (Lewis, Bonnin, Fuchs and Schnyder) are no longer racing.
Of the racers that won a stage in l'Avenir between 2007-2010...
-33 different riders won at least one stage
-75.7% of the riders are currently racing professionally
-90.9% of the riders have ridden as a professional at some point
-90.9% of the riders are still racing at some level
-3 riders (van der Velde, Hartmann and Opryshko) are no longer racing
In conclusion, these numbers don't mean too much since I'm not doing a huge snapshot of their careers, taking into account wins or other major statistics but it can show that there is a
pretty very high chance you will get a chance to show yourself in the pro ranks if you do well in l'Avenir, especially if you are top 10 in GC. I would love to try and pin down exact numbers and see which l'Avenir produced the most number of professionals, the least and which factors exist that can skew the data (course selection, team infighting, selection politics, etc.) but you know...no money and little time.