Develop a gatekeeper that, rather than asking for an age or birthdate, actually tests emotional maturity based on some signal criteria. If you test out as mature enough for the subject matter, you're in. If not, you get redirected to something appropriate for your maturity level.
PROS: Emotionally mature users would be admitted, and others blocked, regardless of age. Advanced tweens and stunted twenty-somethings could both be dealt with appropriately.
CONS: Where exactly do you draw the line? Chronological age may still need to be used as a deciding factor in borderline cases. Also, some sites might find it beneficial to develop a gatekeeper that identifies other factors not directly relevant to emotional maturity--high susceptibility to certain forms of advertising, for example.
Hey, it's just a thought.