You're right, and that's the kind of thing a native speaker doesn't notice because we're used to it. When we say "he is 42" it is assumed that's his age, but if we say "that car is 5" without "years old" the meaning is unclear.
Now that we've cleared that up, how about learning some English? Your question is full of errors.
If you ask to me: "How old are you?", is it correct I answer:"I' m sixteen.", or I need always to say "years old" in the final?
If you ask me "How old are you?" is it correct if I answer "I'm sixteen", or do I always need to say "years old" at the end?
· 3 months ago