If they're that deeply affected by autism, they almost certainly have a caregiver at that point. I imagine that's the solution - whoever has power of attorney for medical decisions would make the call for them. From there, it's simply a personal decision for the caregiver.

If they've got it light enough that they can handle life in the real world (i.e. can enter contracts for renting an apartment, getting a car loan, etc.), then they can give meaningful consent for themselves.