Age inconsistency is generally unavoidable in corporate-owned, long-running shared universes. Unless there's a conscious effort to be strict about it from the beginning, it isn't really worth fretting much over. Some characters will age faster than others simply because it's more convenient for a specific story or simply due to the fact that they're appearing. Likewise they can be aged down just as easily.
Jubilee is the same age as Pixie if not younger. It doesn't make a lot of sense, it merely is.