Sims continue to age while they're in the future. That much is normal since it isn't really a vacation like World Adventures. If your teen had been in the future 8 days then aged up, that would be normal, but you say it happened on the same day you arrived in the future?
My first thought is that it's likely a mod conflict, but then even without any mods added, there are still some strange bugs in the game where the future is concerned. Your teen aging up the day of arrival is certainly not normal if the teen was with young adults or adults.
EDIT: I just had a thought as I hit post for this that may or may not be the cause. I don't say it's likely because the teen was with family, but there's an outside chance that the game code triggered the age-up. Only Young Adults and older can travel to the future unless they are with Sims like parents or something like that. It's possible that the game code saw the teen as needing to be a young adult and triggered the age up. If there's any way to go back to before you made the trip and see if it happens again, that could give you the answer. Then again, it might have been a random bug.
