score:4
My experience with tour groups has generally been guided by two rules of thumb (developed over a fair few organized tours I've gone on):
As a result I've had luck with the low-end options of mainstream tour providers. Examples for your criteria with tour providers I've had luck with in the past, include:
This is based purely off my own anecdotal evidance though, mainly travelling through Africa, Asia and South America. I have a gut feel that the traveller profile in group trips in the USA may err on the older side simply because younger travellers will tend to "roll their own" trip.