For AMHR the stallion must turn 3 the year the foal is born in order for that foal to be registered.
However, that stallion does not have to be brought permanent in order to have the foal registered--as long as the stallion's papers are not expired you can register that foal any time during the stallion's 3 year old year. Robin--there was one or two years where the rule was the stallion had to be made permanent before the foal could be registered, but that was causing too many problems for people that wanted to show their foals & couldn't because the stallion's birthday came too late in the year. It was also a problem for people who bred with a young stallion and then sold him, and the new owner didn't make him permanent. So, the rule was amended to allow foals to be registered even if the sire (or dam) wasn't permanent yet, as long as the temporary papers are still valid. Once Dec. 31st arrives and those temporary papers expire, then any foals not yet registered cannot be registered until the parent is brought permanent.