The only legal responsibility we have is to our children until they become adults.
My Mum was poorly for a long time and for the last couple of years of her life, bedridden. I was still working and towards the end, newly divorced and loving my freedom. I did do a bit to help but I could have done way, way more. I still beat myself up about it 20 years down the line.
When my Dad started to need more help I eventually gave up work to care for him. It was very difficult for me financially and after 10 years of caring for him I became very isolated. But I learnt to resent the situation - not my Dad. He didn't want to be dependant upon me any more than I wanted to be a carer. But we were a little unit in a little bubble.
By the time my Dad passed away 8 years ago I was too tired, too old and too broke to do any of the things I had planned for my retirement. When he died I lost my Dad and my purpose in my life. And even though my life almost 100% revolved around him and his needs, I don't regret any of it, I'd do it all again in a heartbeat. But I do still often think I should have done more.
I have experienced life as a carer and I would never expect my children to do what I did. It does rob you of any sort of meaningful life, and at times can be unpleasant and distressing.
Having said all that, I would personally find it difficult NOT to care for a parent in some way. I loved them both too much. It may be that my children will feel the same one day. Nobody could stop me looking after my Dad - I don't know if my children would feel the same!