It is correct.
it is not correct English, you rather say still
By or at a certain age are both correct, with slightly different meanings. We say by age six when we are thinking of a succession of years; but we say at ten years of age when considering that one time.
It would be more correct to say, someone who.
It is correct English to say decent skill but not decent ability
i have been thinking of you
It is correct English to say "He went off on a tangent".
no it is not correct to say you are not for sure instead you can say in proper English that you are not sure about something
No. In English we say "How old are you?"
Yes, it is correct to say out of compliance with. It means out of help with in English grammar.
No, it is not proper English to say "on tomorrow." The correct phrase is "tomorrow."
no you should say "Did you get an A on the English exam?"
Correct is the same as it is in English. It means that it is free from error.