for a teenager, I definitely wouldn't recommend Ros, unless they're already very deep into Linux, c++, build systems, etc - I'm sure there are kids out there that can handle it, but I'd say most would find Ros endlessly frustrating. I think it makes grown men cry.

robot wise, I'd recommend something they can program with a raspberry pi and python, if they have some light background in programming. I had a quick look on Amazon and I have no affiliations with these, but maybe look for something like this: https://www.amazon.co.uk/FREENOVE-Raspberry-Tracking-Avoidan...

note this one doesn't have either a raspberry pi nor any batteries. you might find a better one.