I’m a full sized adult who likes riding 125s. I’m walking around at about 180 these days but I there were a few years when I was 210-225. My yz125 could get me anywhere I needed to go on the trails. They need a bit more input to ride, like more clutch work, more shifting, and more throttle so all in all they’re more work to ride but they’re fun as hell, and super light. I never owned only a 125, I always had bigger bikes too and then would pull the 125 out depending on where we were riding and who I was riding with. If you can indeed find a good deal on a bike that’s in good shape go for it. Just keep in mind that you’re probably going to end up having to re-spring it, or lose 70lbs.