Old enough to have finished growing for sure. I don't like to use studs that haven't gone through at least saddle training - meaning they must be good on the trails, at least to consider them. I don't like the old pasture studs that never go anywhere or do anything but breed like crazy. I like them to be fully grown so that you can judge his conformation correctly, and trained so you know how his temprament is. Around here, it seems to be around 4 or 5.
Ask Often, Demand Nothing, and Reward Generously.