According to a CM on my Disneybounding Facebook group, new rules start (are starting enforcement) today, and costumes are allowed for 14 and under. For the bounding/cosplaying groups, there's considerable strife right now about what will be enforced.
Basic rule is this: Will a child be confused in any way by the person in the costume?
If there's no way they will be confused (i.e., it's clearly another child playing dress up), then it's probably okay. However, if the costumed person is looking a bit too much like the character (thinking of a well-developed teenager in a costume), then there's a fair chance they will be turned away at the turnstiles.
And there apparently is now a ban on firearm-like toys being used as part of a costume in the parks as well.