I don't think age restrictions are particularly useful, but they are there for a reason. When talking about mature topics, it is important to take into account the maturity of the individual, which is hard to assess. Children may also hear things from their classmates if not from their parents, older siblings, etc.
However...
When I see 8 yr-olds c...... 2 reply
It's kind of random, but lately I like reading yaoi that have the "forgotten first meeting" theme. As in years pass by until the seme and uke meet again, but they're "pulled" back together. reply