how do you travel to another city?

Usually by bus or train.

What do you do if the city has high slopes making walking and biking too hard?

Walking is good for you, biking is not too popular in cities with slopes, but electic bikes are changing that.

Or how do elders deal with what other citizens would take for granted in terms of mobility?

There is definitely less mobility, but that is part of getting older isn't it? Usually they just walk a bit slower and use busses and taxies.