Is 15 old enough to be in a serious relationship?

If you consider yourself mature enough then yes. I've been in a committed relationship since I was 15, I'm 19 now and I'm still with the same guy :) I'm pretty focused in school and I always made it my priority so it did not affect my education and I did join after school activities, one of them is going to summer school at Stanford university, where I would take classes, prepare for the SAT, live in the dorms and take math classes that I would take next year. So yes, you can do it, but only if you both are committee to work out problems and stay together.

I was 14 when I started my current relationship :) I'm 17 now, still with the same guy.

So yes, it can be as long as they're both somewhat mature.

Meh. Every person has a different level of maturity.

If you think it will be a "I love you forever, we'll marry, and never break up" situation, then no, you're not old enough for a serious relationship.

Relationship? Kind of an iffy, but yes. If they are both responsible.

As long as they're not obsessed with each other.

I mean, they're going to break up anyway.

that's cute