"I have 20 years" or "I am 20 years old"?
I have 20 years.
I'm 20 years old.
The rule
In English you ARE a certain age, you don't "have" it. Say "I'm 20 years old" or just "I'm 20," not "I have 20 years."
Many languages use "have" for age ("I have 20 years"), but English uses the verb "be": you ARE a certain age. This is one of the most common first-language translation mistakes.
More examples
She has 30 years.
She's 30.
How many years do you have?
How old are you?
My son has five years.
My son is five years old.
How to remember it
Age uses "be": I am 20, she is 30. And the question is "How old are you?" — not "How many years do you have?"
Frequently asked
Why "be" and not "have" for age?
It's simply how English works — age is a state you are in (I am 20), not something you possess. Many other languages use "have," which is exactly why this slip is so common.
Can I just say "I'm 20"?
Yes. "I'm 20" is natural and common; "I'm 20 years old" is slightly more formal. Both are correct — just never "I have 20 years."