there was a thread on this about a week or so ago? I am sure I saw someone said it can happen under a year...do a search on your title and I am sure you will find the original thread
i was told by my vet that once the ball are out then anytime soon when he realises what its all about so if hes interested beware, but really any time after weaning. if a mare is in season in his face the penny may drop quickly.
My (now sold and gelded) welsh D tried it at 7months with his mare field partner a maiden learning baby sitting, he was soon moved out of that field and into the geldings!! naughty ponio!
Yet my very large warmblood 4yr old didnt have a clue!! he has since been chopped for his lack of interest, ooh the grass was far more inviting!
Colts -occasionally- can be fertile enough to get a mare pregnant at 6 months old. However, the majority will certainly be able to get a mare pregnant from 10-14 months of age. However, there are never any hard and fast rules in equine repro...
Put it this way - as you can't gaurentee anything with horses, my filly isn't going with her playmate until he has been 'done' just in case.....she is 9 months and he is 6 months.