I started dating when I was 12, but by that I mean that I was with a girl with no sexual contact, called each other boyfriend and girlfriend, and we would go into the town centre together alone, maybe going to the cinema and what have you. That sorta dating, not taking someone out for a meal and going back to their place 'for coffee' (a 12 year old doing that these days is rare, but it does happen).
At the time of my first girlfriend, my friend-at-the-time's mother found out I had a girlfriend, and berated me for it, saying I was too young; my mum, on the other hand, was fine with it. She was checking her son's texts when he was 16 years old to see if he talked to girls, so...yeah, her criticisms of my choices didn't really bother me. Her son had his first girlfriend at the age of 17, but seeing as I hadn't spoken to him for a couple of years, I don't know how his mother felt about that.
Proper dating, where we go for meals, cinema, and the like? 18 years old was my age at when I started doing that. Been dating the same girl for three years now.