When you say RS Grand Tour of Spain”, do you mean Road Scholar or Rick Steves (this forum)? I ask because Rick Steves does not have a tour called that. Rick has several tours on this website that you could look at.
In answer to your question about ages on tours: I do not have personal experience with Road Scholar, but I understand that they have mostly older people on their tours and may go at a slower pace (years ago they were called Elderhostel, aimed specifically at retirees). They have an excellent reputation, but I’m not sure I would take a teen.
I have been on 10 Rick Steves tours. Most have been all adults, mostly 45 years and older (much older - I am 73!). But, depending on the time of year, some have had teens and young people. Right now I am on a tour that has a mother-daughter (junior in college) and a grandmother-grandson (recent high school graduate). There is no guarantee, but you can call Rick Steves and, if a specific tour already has some people signed up, they can give you an idea of the makeup of that group.
Also be aware that rooms that accommodate three people are rare in Europe, so think about whether you would be comfortable with splitting up in some configuration. This would likely be true with any tour company. I believe that Rick Steves requires children under 18 to share a room with one parent. You should verify this policy with any tour company you choose.
You might look at Rick Steves FAMILY tours. They do not include Spain, but I’ve heard they are really fun and attract many families with teens. And My Way tours, as suggested before, might be a good match.