The only one I had neutered after he'd attained more than 6 months was our first Lab, Brutus. Bru was....shall we say.....rather large.....
I don't remember the exact amount of time it took but Bru ended up being smooth back there.
Dan was neutered at 6 months and underneath a LOT of fur, there's still a bit of skin. Dan is now 5.
Doesn't help, does it?