My ds had this, but it was picked up earlier, at his 6 week check.
Testicles (apparently) can be tricky to detect as sometimes they can do a disappearing act, which may be why it wasn't picked up when he was tiny. Is it one testicle, or both that aren't down? I think we were referred at about 4 months to the surgeon, and they just kept an eye on it every 6 months or so.
If it doesn't come down by itself, the standard age to do the op is about 16 months. Lots do come down by themselves, apparently, but they were pretty sure my ds's wouldn't, as it was stuck somewhere in his abdomen.
The op itself was fine, just day surgery. He had to be fasting , which I thought would be a nightmare, but was actually ok, as he just went very floppy and sleepy, so I could walk up and down with him until it was his turn. They put an anaesthetic gel on the hand before inserting the needle, and I was allowed to stay while they administered the general anaesthetic. Tbh, that was the worst bit, as they look so small and vulnerable when unconscious, and he went a bit blue as well, which is apparently not uncommon.
After the operation he was fine, woke up, downed a bottle of milk and went straight back to sleep. He had one small incision in his abdomen and another in his scrotum. Both were done in a natural skin fold, and are now invisible (he's nearly 6). After the op you have to keep the nappy very loose and not do up vests between the legs. They advise you to keep dosing him with calpol/nurofen to prevent any pain building up, which we did. It was all no problem, frankly we've had worse reactions to vaccinations.
It does seem to be pretty common, and something paediatric surgeons are very used to dealing with.
hth