LOL.........a colt usually starts to become fertile towards 18 months, however, this is not always the rule of thumb and they can become fertile earlier than that. My advice, when they become yearlings run them with other yearling colts or geldings, that way you wont land into any unwanted trouble.......
my colt was mounting my nieces gelding around 6 months, hes very randy now. will be 2 in may.
colts are capable of covering a mare normally around 9 months onwards, but i have know 1 that covered a mare at 7 months.
when both testicles have dropped properly they are capable of covering a mare.
There's a rather lovely traditional coloured colt (18 months) in a nearby field! Would make a lovely hubby for daughters pony. Owner says I'm welcome to use him if I like. Mmm....