I agree with the above posters that you need a notarized letter from both parents (or proof that there is only one legal guardian). Even one parent traveling with a kid can encounter a situation where they wish they had a permission letter from the other parent.
On the other hand, I flew overseas by myself many times at ages 16 and 17 with no problem. I think most of the problems occur when there is a younger child involved. But easy to get a letter now and very hard to wait in an immigration room in Japan while the parents try to find a notary at 3am so they can fax you a letter.
From a United perspective, the risks of split PNRs are the same as always -- you'll need human intervention and patience during IRROPS, but they will eventually put you on the same flight. And slightly higher chance you'll get seated apart if there is an equipment swap. Overall, if you want to split it cash / miles it's worth doing that and dealing with the split PNR issues.