1. I wouldn't say it's a waste of time. It's practice. You learn more about yourself and about other people. Just like what has been said above.
2. Why does a relationship have to be about marriage? At a young age it's just about being as "close" as you can to a person you feel very attracted to.
3. I agree with your point about planning break ups months in advance. I despise people who treat the emotions of others as secondary to their own.
4. A relationship can only narrow your friends if you allow it too. If your friends can't respect your decision to date someone they are not good friends. Like wise if a partner can't respect who you decide to spend time with they aren't very deserving of your attention.
5. Same sort of thing applies to the supposed limit on your activities. If they won't allow you to do certain things on your own than they aren't your partner in the relationship they're a tyrant.
The truth is though, even further than all of this, age does not determine maturity or even if you're interested in relationships. Those depend on your own development and personality. Some 13-year-olds are about as mature as a toddler others are more mature than the people running our countries.
If I use myself as an example. When I was 13 I was very much interested in relationships, not just date and dump, but long term relationships. I had a friend who wasn't even remotely interested in either. So to some it up when it comes to relationships it should be maturity that is the deciding factor not age.