I'm old but I have done 8 RS tours, 5 of them as a solo traveler. My tour buddy on the 21 day Best of Europe was 30. I've been on tours with people in their early 20's. They are perfect for me and I have always felt very included. I often strike out on my own for meals as I am vegan and that just adds a nuisance layer that is hard to inflict on others, lol.
The most strenuous tour was the 21 Best of Europe where we did go 10-12 miles a couple of days. In general they are pretty active and there is always an option to add more. The last trip I did, GAS, one Dad/daughter (about 20 years old??) combo were working on 20,000 steps a day and if they hadn't gone that much by dinner, they were off exploring to add some more steps. I know they would have welcomed ANYONE who wanted to join them!
I have not done the Eastern Europe tour but the reviews seem to be really good!
If you happen to live in the Northwest, Rick has some travel festival days where guides and others come in to give presentations. They are free and lots of fun. The ONLY problem is that you will want to go on ALL his tours after you get a taste!