I haven't been on an organised coach holiday but have done many day trips that way, both in UK and to mainland Europe.
Is it mainly the fancy coaches and on-board facilities that appeal to your DD? If so, I'd advise checking different companies' websites and brochures to see what their fleet of vehicles is like, as it can vary quite a lot.
Yes, you're likely to be amongst the youngest on board, but again that can depend what the destination and itinerary is. PP makes a good point about the hotels companies use sometimes being out of town etc.
If your DD is also interested in the logistics of coach travel - all the destinations, routes etc - then I'd echo PP's suggestion of putting together your own itinerary, say with National Express or Megabus, and if possible using one of the major coach stations, like Victoria Coach Station in London. I know there are other major hubs but that's the one I know best. She'll see coaches of all different types going here, there and everywhere, including ones heading to mainland Europe.
That kind of travel would likely be more cheap and cheerful, but facilities should still be okay, and there'll be a real mix of passengers. Each to their own, but I tend to prefer that kind of coach travel as I travel alone and find organised trips can be a bit cliquey, though people are usually very pleasant.