Forcibly cutting his claws ain't gonna win you any brownie points either.
Seriously though it doesn't matter what age, you can get sliders to get used to you by interacting with them and picking them up. Giving them a special treat when your done too helps as well.
I'm taking care of a wild snapper right now, and the first day or two he was seriously out for blood any time I picked him up. Two weeks now, with me regularly annoying him (he's got some shell rot, and I've been tending to it) he's totally used to me picking him up and will take food from me.
Personality wise some are crankier than others, but with regular interaction within a week or so he won't even hiss at you anymore when you pick him up.
