What Laggard meant is that you have to be a pro rider riding for a pro team and that pro team has to be one of those invited to the race. That also means you have to spend a large amount of the year in Europe since US teams that don't race in Europe don't get invited.
I'd guess you have to be 18 to sign a pro contract. No upper limit but by the time riders hit the mid to upper 30s they're usually done.
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