There is no age that is too young, assuming they have knowledge of numbers, counting, can play turn based games, and can understand which
hands beat what.
Well, I guess there are ages which are too young, but I have taught kids as young as 4. With almost all kids, no matter their age, I start with 5-card stud. Usually fixed limit, although we sometimes move to PL or NL when they understand those formats and want to go back and play 5-card stud again.
5-card stud is a perfect teaching game. It is great for teaching the rules of basic play: people act in turn, moving to the left, choices are check or bet when no one else has put money in, or fold, call, or raise when someone has, you use the information you do know to make guesses about what you don't know (which is only one card per player), you can make a bet to make people think you are stronger than you are (
bluffing), and how to compare 5 card hands when there is a showdown. There are limited decisions (compared to other forms) and hand-reading is trivial because you only have 5 cards and can't exchange any.
Then I teach them draw and eventually 7 card stud games. Shared card games (hold'em and omaha) as the last formats I teach kids. But, I almost always start with 5 card stud, to get the basics down first.