I personally wait until my teens are at least two days from adulthood. That way they have time to get their skills maxed, rack up scholarships, etc.. while enjoying their teenage years.

Maybe he means they had a wish to go to uni, seeing his question is about when she can send them.

My teens are all over the place depending which hood it is. I have a double aging mod put in mostly to give toddlers and children longer life spans, but it also gives teens 30 days. Now as teens are my least favourite age I normally don't want them to be teens for that long. In my normal hood teens have to meet certain criteria to go to uni which they need to meet by day 10. In my neanderthal hood they get to be independent teens instead. In my legacy hood I send them to uni on day 10 regardless.

Why, why didn't I think of this when I was playing one household that crashed, losing a sim week of time?

I never have a set time to send my teens off to college. Some, if I'm bored with them, get sent asap, others I milk it for all it's worth until the day before they're supposed to age up, then send them. More often than not, I would run out of time before having to send them to college, so I added in the mod that Jo mentioned, which gives teens 30 days. Now I feel I can play a more relaxed game, letting them enjoy their teenage years instead of making them skill all the time. Heck, now they can actually find that teenage sweetheart!