Hi Emma,
that's a question that has no answer. It simply is "J'ai vingt ans." if you want to say your age. In French you have the years, you don't are them. It may be a small consolation to know that it must be equally weird from the French learning English.
-- Chris (not a native speaker).
Bonjour Emma !
Like Chris said, I think it's a question of point of view :)
In French, we consider you have [gone through] these years, not that you are "old" of that many years.
That's also why you can never just say "J'ai vingt." but always to mention "ans", because you could have many different things :)
I hope that's helpful!
Bonne journée !
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