@Cybersnow You are not very tall, so you would prefer a step through frame, or maybe a dropper post. You are 80 but in excellent health.
If you are in excellent health, you do not need a step through frame. A step through frame requirement will reduce your choice of bikes considerably, try your best NOT to go down that route. Once you get biking you will regret the frame choice; it will be holding you back.
You might be 80, but you are not over the hill yet. Your bike will be carefully chosen to be of a size to suit your size and body geometry. Therefore unless your hips are at risk of dislocation, you will be able to get on it. Drop the saddle before throwing your leg over, raise it before you set off; it's what I do. If the saddle is still too high, or your preferred mounting leg is stood in a hole (it happens), then tilt the bike, put your leg over, raise the saddle and go! It's what I do.
When you go to a bike shop or try other people's bikes for size, have no embarassment at all about your mounting technique. You are 80, not 18! I'm on the cusp of 75 and I could not give a toss about what others think about how I get on a bike or how I ride. I don't wear my hearing aids when I'm riding so I am unlikely to hear any wise cracks!
If you are fit enough and flexible enough to get off the floor unaided and without having to hold onto a nearby object, then you are good to go mountain biking. Go Ride!
