(NSFW) When is a normal time to begin masturbating/etc?

My brother spends a lot of time in the bathroom (he’s 15), and I was talking to my mom about it, and how I think that he’s been watching porn and such. I checked his history, and he wasn’t, but he was being really secretive about what he was doing on the computer (apparently he was just watching movies like Superbad and The Hangover).

Mom said that her brother had Playboy under his bed when he was around 15 or 16… she said it was normal, but I’m not so sure.

What do you think is normal for masturbation/porn/that sort of thing?