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.

My initial inclination is to suggest that "I am of legal age" is the correct one. I do not know exactly what you are trying to say, but people DO say "I am of legal age." [for example at 21 years old, one is of legal drinking age in Michigan.] I am full of age doesn't really work. Full of is used to indicate something that has a set limit to it. Like how full is the gas tank? It is completely full [of gasoline]. It is 3/4 full. Or it is something that can be put into someone or something. Like is he ever full of ideas today. Your age does not really come out of you or go into you, and it doesn't have a constant capacity that keeps being refilled. Yo soy mayor de edad LITERALLY translates to I am full of age, but the actual meaning of the phrase in both Spanish and English is that one is of legal age.

I am of legal age is appropriate in American English. If I am of full age is appropriate in another Anglophone area, I would not be able to tell you, but it is not in the states, at least colloquially speaking.

I think you are meaning that a person saying Yo soy mayor de edad is someone who's over 18 years old. So, in English, you should say: 'I am over 18' or also 'I am of age'.