I would say first look at what their high school math trajectory would be.
If they do Alg 1 in 7th, Geometry 8th, Alg 2, 9th, Pre-calc 10th, Calc AB? BC? 11th and then Calc BC ?Multivariable in 12th? DOes your HS offer that? or would they have to go to the local community college? Would that be logistically feasible?