If you feel unsure about someone’s age, it is always good to check.
At 15, intimacy would clearly be considered illegal.
15 could be considered Legal under a "Romeo and Juliet" law.
So since you're 15, for example, you could not legally consent to be with a 21 year old.
You're confusing whether it is illegal, and whether it can be proven. Whether anybody can prove it or not depends on the facts and circumstances of the case.
A 12 or 13 year old can consent to sexual activity with a partner as long as the partner is less than two years older and there is no relationship of trust, authority or dependency or any other exploitation of the young person.