Underage at least is something that has had a vehement push against it for longer than that. That came with far more strict enforcement of laws regarding child porn and people being paranoid that writing porn involving an underage character could get them arrested and labeled as a sex offender.

Even on LJ people had serious issues with smutting with underage characters, which is why most sex games tended to require underage characters be aged up. People would also freak out if they found out they played smut with someone under 18.