When I worked in Care homes, it bought home to me how well my Mum was, she is now 86.
Other people aren't as well and a hostel system for someone with health problems, wouldn't be appropriate.
The bedroom tax applies to people at 55, but many would struggle with some types of accommodation offered, for instance.
Governments have to plan a Care/Welfare system and set funding levels, Charities have to pinpoint their demographic.
So the age setting of an older person at 55, is appropriate.
""I think there is a gap in the market - for affordable housing suited to those with, in particular, restricted mobility. It doesn't need to be age-based, IMO.""
We have blocks of low rise flats that are age restricted, there are enough issues with visiting Granchildren etc, without a mix of families and people who want a quieter way of life.
We had a development build with mobility restricted younger people in mind, which gave some appartments their own garden and were deliberately close to schools etc.
These have worked well, for the five years they have been built and have improved the quality of life for disabled parents.