Hi @sunwisecircles - You probably already know this, since you said you have aged other Sims, but just in case - the cake is not going to have an option called "age up" or anything like that. To age the sim, you have to have him blow out the candles. That is the trigger for aging up. Please forgive, if you already know this, just wanted to be sure.
@SheriGR is giving you great advice in the event that you don't have the option to blow out the candles when your teenager clicks on it. Hopefully she can help you get it figured out soon. Happy simming!