Each stallion has a max of 20 vials. Thats all
At this point of time, you can collect the semen at 33 years . . . however its highly recommended that you do not. It will eventually be fixed that you will not be able to collect semen past the age of 23 (maybe 25, with the new bonuses).
Anyway, it doesn't matter when the semen is collected. What matters is the stallion at the time the foal is born
