You might also want to say of full legal age in some contexts. For example, in some parts of the United States, you can vote, can serve in the military, and have most rights of an adult at 18, but you cannot buy alcohol until you are 21. So saying you're of legal age can be a bit vague. Of age to do what? Buy a car? Inherit property? Be tried as an adult, not a juvenille, if you commit a crime?
You can also simply say you're of age, if the context is clear. "You want to buy some beer for the party? But aren't you underage? "No, I'm of age now."
However full of legal age is incorrect.
Subject,verb,object is the missionary position of the English language.