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):
- The "gap year" style of trips will generally be as cheap as possible, so tend to come from a few dedicated providers, e.g. the famous Contiki tours.
- The "pensioner" trips will often be the more expensive options on a given tour.
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.