I'm 11 years old or I was 11 years old

both are correct sentence first one is present tense's senetence and 2nd one is past tense's sentence.

Both are correct but both have different meaning.

I'm 11 year old. i was 11 year old. "s" is not used with year here.

I'm 11 years old. - present tense, it means, "you're of this age now. I was 11 years old. - past tense, it means, "you were of this age before but not now.

hmmmmm