My colt had them almost from birth. Probably within the first couple of weeks or so I could see them. My friends were impressed! But my vet actually said about 1/2 of the colts are born with them already down, so I guess it isn't all that uncommon. I got mine gelded at 5 months.
It is possible they are both there and you just haven't seen them both at once. I have heard that they can drop them and retract them and sometimes you will only see one at a time. I know I have neighbor's who have had colts gelded and when the vet came out he could only spot one, but he would go ahead and do the surgery if the owner said they have seen both. I think once they both have dropped they can't go all the way up into the abdomen anymore, but sometimes they hide and you don't see them.
Hopefully he isn't cryptorchid and will show them soon!