Translation:I am not young.
Can it be I am not a boy?
No, "fiatalok" in plural, is sometimes used to address "young ones", but "fiatal" is an adjective.
you might have mixed up with 'fiatalember' which means something like 'young man'
Could fiatalember mean 'young person'?
Can 'fiatal' be translated 'youthful' or is it strictly related to age?
It is age related. Youthful would be translated to "fiatalos".