This happened the other night while I was playing my personal challenge of "how many babies can a married couple have?". I had twins aging to YA but the first one didn't age. The second one aged just fine with the same cake. I decided to move them out anyway, because mom & dad were going for more babies. Then I went to the twins new apartment and aged up the male. By-the-way, I don't have the new Paranormal pack in my game. I suspect the latest patch did this.
I'm kind of nervous about aging up the next set of twins, since my Sim parents have 8 children and they are 4 sets of twins.