Tried an E-Mountain bike a couple of days ago. Amazing what it does on the hills. You might be able to ride until you a 100+
Does it help with balance? That's what eventually stopped my father from riding.
Recumbent trikes can definitely help with the balance issues, and I think they do come in MTB versions as well. Presumably wouldn't be so good on narrow single-track!
Helps with balance on steeper sections, but the bikes are heavier so may not be helpful overall.
My e-bike is pretty lightweight, rides pretty normally with all the power assist off.
It's coming, but not yet. :-)