'I think it's right that older people downsize to move in with the younger ones.'
That's nice for the younger ones - what have we done to deserve that? Perhaps we don't want our parents/in laws to live with us. I would have cheerfully looked after fil, but sadly, he died last month. I will not however, be offering to house mil any time this century. Whilst I would have my mum to live wit me, we have done it before for six months when she retired and was house hunting near us, but that was the time limit for our respective endurances.
As for the care funding discussed earlier - having just had my fil in a care home, there is very little state help available even before one looks at the value of a house. If you have capital over £23,250 you pay all your fees (except the NHS contribution if in a nursing home) and once you reach £14,250 your capital is disregarded, much like with HB. It is unrealistic and unkind to expect an elderly person to cope with putting their spouse into care, sort the finances for that, which are long and complex, and then have to deal with selling, finding somewhere else, and sorting the detritus of a lifetime out to boot.
Iggly, by the time you're mid 40s you may have changed your mind about handing everything over to your son; mine will be 25 in exactly 9 years time, and I don't propose to downsize to give him a house deposit. He would feel guilty. We will give him help through uni and he can live with us for as long as he needs, but I am not Italian or Spanish, so he needs to be more or less moved out by his late 20s.
Of course, another way of having care is having enough space to be cared for at home, either by a live in carer, or a team of carers - you still pay obviously, but it is not so much as a home and you stay in your own surroundings.