When our sons reached about 120lbs we moved them up from their 80's. One was a very skilled MXer and at about 110lbs we had to intall stiffer springs in the bike because he was outriding it.
I'm barely 5' 10" and full sized bikes work well for me. As mentioned, a full size bike can be lowered slightly (suspension mods), the seat foam can be shaved (elec carving knife), or the subframe can be shortened a little.
At your age, size, and the fact you're looking to trailride, I'd keep my eyes open for a used Honda XR250, XR400, CRF250X, Kawasaki KLX250, KDX200/220, Yamaha WR250F, Suzuki DRZ400... there's a lot of choices...