My own opinion is that the younger sires mares produce better ofspring
first crop is usually hit and miss but there 2nd 3rd 4th crops are better with younger sires/mares
just my own opinion but have played this game rather alot

too much
You can have the best of both worlds you can export your young horse to the gamepool
and send him to stud and bring in the copy to race a full career
I dont know if you know how to do that so give me a shout if you need instructions