I'd have said Ulfric looked to be between 40-50 to me, Saadia around 30, but I'll admit its just guesswork, hardly concrete.
As for somebody in their late teens being able to betray the city how?
Even if noble I doubt they'd have the authority to order the gates to be opened or order troops away from a vital defense point.
Look in history for the answer to that. A person in their late teen's in that type of society is not like the late teen's of today. Boy's a young as 13 acted and were concidered advlt's,Girls were given away in marriage, it's not really until the 1900's that society decided that one wasn't an "advlt" until 18 or older.
If your "Lord" or "Lady" was a teen you still listen to what they said to do.
Because they don't actually give a good story on how the city was betrayed we don't know. A 13 yr old girl given in marriage to an old fart may have "betrayed" the city out of spite. ( and history say's their are girls even younger than that given in marriage) The betrayal could be as simple as running away from an arranged marriage, that brought together two great houses or kingdoms.( provences)
JMO that even a young teen could in one way or the other betray a city.