How old do you have to be to take MMA lessons?

It really depends on what you mean by "mma lesson".

If you're talking about a seminar or something, they would most likely expect you to be at least in your teens. For an MMA school or something you can be as young as 6 in some cases (the mma school i train at has a youth program for several different age groups, one of which is ages 5-6). Just call the place you are looking at training and ask.