He was four and a half months old (lol... we gelded him very earily because he was getting a little too much to handle... we were origionally going to wait until he was two)... It didn't take long to heal, but he did swell up pretty badly when I was sick and couldn't exercise (walk) him for a few days.
I'd guess that the healing is about the same no matter when or what age you geld... ''watch him about two weeks'' was what my vet told me... My boy never acted like he was hurting or anything either, even whn he swelled up.