Given this gray area of what is "mature" and "legal" or "Illegal" or "Imoral" this is not going to be settled. but if I may.
1) Pornography is always considered a VISUAL Media: print, photos, drawings, etc
2) Most places count Child Porn/Pornography as illegal, even Japan has anti-childporn laws despite the drawn lolita art. (lets not get into this)
3) Depictions of violence such as movies, drawings, etc is not illegal. Photos of RL Violence is (most likely) illegal if not used in news/education/etc.
4) Depictions of Beastiality is subject to local laws but most do not count drawn or animated depictions to be the same as physial acts.
5) There are always victims of crimes, ALWAYS. but there are common sense levels (example: Theft is a less victimizing crime than Rape. Jury's still out on if Murder is worse than Rape)
6) Ageplay itself means Roleplaying an age you are not. Roleplaying should never be considered Illegal as long as both are consenting adults (local laws apply for real life, 18+ for SL) behind closed doors.
7) Linden Labs does have a closed bedroom door policy, most reports of sexual ageplay is because of people RPing it where people can see/hear it. They do not send spies out to check on the millions of residents constantly. They only go by residents reporting it before they check the logs.
If I'm mistaken please note by using the number and expain in a civil manner why I'm mistaken.