OK, I've just had a look at the Tour de l'Avenir which is something I've not really done much, if at all before.
I have learned some stuff, which I'll share for the benefit of any other ignoramuses (ignorami? or is that the art of stupid paper folding?)
There's a Women's tour de l'Avenir going on right now (actually just finished). Not knowing that made me feel very ignorant (not a new feeling)
The Tour de l'Avenir has its own white jersey Youth Classification. Which, as far as I can tell is U21 (ie 18-20 inclusive)
All of the final top 10 except for Luke Tuckwell (10th) were also in the Youth Classification. This is either because the upcoming generation is exceptional, or because really good older riders aged 21-22 are off riding in senior competitions.
This is quite an interesting summary:
https://www.cyclingweekly.com/news/what-exactly-is-cyclings-tour-de-lavenir-and-why-should-we-care