An age header is not the answer. Why should a site have to decide what content is appropriate for a 18 year old and what content is not? Who is qualified to make that decision for every 17 year old in the world? Do they know my 17 year old? Do they know the rules in our home? What if I'm OK with my kid seeing sex-education stuff, but some lawyer at Wikipedia just decides to tag sex ed articles as 18+? Now I have a shitty choice: Open up the floodgates of "18+" to my kid, do it temporarily while the kid browses the sex ed sites, or not let the kid browse them.

Letting a company or government decide what's appropriate for what exact specific age is fraught with problems.