Well, to begin with, sadly, disability and infirmity is not restricted to the old. There are a significant number of people of all ages, including children who benefit from this equipment.
Secondly, the people in these ads are there purely to indicate how these products work, they are not there to 'sell' the products in the way that breakfast cereals will advertise with happy excited families eating their breakfast and then going out for a happy walk with the dog, all gurning faces and white,white teeth. These models are indentikit, and modestly dressed in neutral clothing, swimming costumes, huge towels or whatever and they will always be opening the bath door, sitting in the shower or whatever, showing how it could be used.
If they used old and infirm people, it will be off putting for younger disabled people and people will focus on the models infirmity not the utility of the appliance and disconnect if the disabilty is not theirs,
Their attitude might be, 'Oh that person can hold a rail so it will be no good for me because I am in a wheelchair.' or vice versa.
It took me a while to realise this, but when I started to think about what models would work I realised that we would all be looking 'for someone like us' when what is needed is models who are like no one who would use them so people dissasociate form the models but engage with the products utility.