Yes, that's correct.

Yes it is

" I am 20 years old " is correct and it is how you would say it in a formal setting. The reason is because you should avoid using contractions such as saying " I'm 20 years old ". This is still correct but less formal

Perfect! 🙋‍♀️ That is correct.

Yes we can sey : I'm 20 and say I'm 20 years old. all true