I'm mid-40s and got the odd smack on the bum from DM. DF never smacked, interestingly enough.
I don't think it did me any harm, as Mum was otherwise a lovely, warm, caring mother, and she only did it when we'd been massively annoying.
We got threatened with the wooden spoon from time to time, but it never actually materialised. All our friends got the odd smack, too.
But much as I don't think it did me any harm, I'd never lay a hand on my own kids. Ever. I remember DH saying pre-kids that he wouldn't be averse to giving them a whop if they deserved it (which I said wasn't acceptable to me) - but he has never hit them, and says actually having them, showed him how much he simply could never actually do it.
Times have changed and it's not acceptable as a discipline method any more.
It's weird that I don't have any bad feelings at all towards DM over it, but would never do it myself.
I think it's the context - it was normal then, and carried out sparingly within an otherwise loving, nurturing, parent-child relationship in which I felt unquestioningly secure. Whereas it's just not acceptable any more.