I have a mobility scooter similar to the one linked to in Joe's post.
It is quite light-weight and takes apart yet still manages a brisk 5mph down the pavement.
I bought it secondhand from a local shop for £350 which I thought was very reasonable as they threw in a brand new battery.

I can take this small scooter away with me in my camper as all the bits fit across the driver/passenger seats when I need the rear for habitation.

I feel the main problem faced by many elderly people on Mobility Scooters is that they have never been drivers - so lack sufficient control of the vehicle. I certainly think that a rudimentary test should be compulsary, at least for those using a road-legal scooter.