Hi,

You can get 24" wheeled single fold mountain bikes. Towards the budget
end of the market and would need some mechanical and maintenance skills
to keep running sweetly, and being budget unlikely to be particularly light.

Stiill a good fit is paramount more than anything else, and decent junior
/ teenage bikes do exist for the smaller person if you look hard enough.

Forget 20" off road is my basic opinion, especially fully folding.

rgds, sreten.

single fold and small will generally fit in the rear of a car.