How can I answer this question? It's fine to say 'I'm fourteen'. That is the most usual way of answering the question. You can say the long version if you want, but you don't need to.

"I'm fourteen" is fine in an informal and contemporary conversation. The 2nd phrase would sound peculiar (to me) these days - but still in use :)

I am 14 year old ........is a right answer

Yes,You can say I'm fourteen And you can say fourteen it's also right