I think it depends upon the environment/upbringing of the person and his tenacity to play around with a highly speculative game. I've known some kids here in our country (Philippines) as young as 12-13 years old to gamble against adults, especially those who were born in the slum areas. They are usually children of gamblers as well, and they learn at an early age how to play against the psychology of an adult in a game in order to earn a bit of cash.
12-13 years of are is very young to start gambling in my opinion. In my country you need to be atleast 18. I remember when being 16 that I really wanted to go a casino, but even with my parents it wasn't possible. The only thing I could do was play scratch cards at home. With hindsight it was probably good that I couldn't go gambling because the risk of getting addicted is much higher in younger ages. If the young gamblers where supervised by their experienced parents than it might be okay.