It is a societal norm, so in my society I am right, in their society they are. The societies we live in shape us to such an extent that a 15 year old might be capable of consent in Mexico, but a 17 year old might not be quite ready to in America.
At some point we simply have to say that a person, ready or not, must take on the responsibility to consent, or be deemed forever unable to do so. Perhaps we should have a better system for determining, perhaps we should do a 'coming of age ritual' of some sort that allows us to determine on an individual level which people should be considered ready, but such things are considered too cumbersome for a society our large.