I would suggest that the age group that should be approached should contain those who are most impressionable, if you want to make an effect. We will be impressionable for various reasons at various ages. But I see no reason why education regarding healthy living shouldn't begin right from the start, in kindergarten.
An American politician was objecting to sex education and sex roles coming out in some kindergarten material, which was meant in a very kindly manner, but he misinterpreted it. Perhaps, if we're going to entrust kindergarten children with that kind of information, we can at least tell them how to eat properly.
As we head into teenage years, another excellent opportunity is what is cool and what isn't. For a long time, when I was a kid, it was smoking, and now it is not. So those changes about what is socially acceptable are particularly important to a teenager, and not so much to a kindergarten participant. It varies with age, but I don't think there is an age that is too young in the schooling system.