There can be many reasons why we fear ageing, which have been mentioned.
The fear of becoming frailer, ill, dependent on others and suffering (not to mention dying) can be very much part of it.

If we don't yet fear these possibilities, it is often the regrets about what we haven't done or had. This is a big issue with me and I don't deny that becoming older has increasingly become an issue. In some ways I managed to do a lot of things I really wanted to do such as different degrees, jobs and a profession, some travelling, reading an awful lot etc., but I find it hard to accept the holes in my life, ie the absence of children or at least a child and the absence of a partner during most of my life. So, for me, in recent years, getting older has been very much tied to biology and sadness about having seen my reproductive years pass without having made use of them.

Ageing and getting older - not an easy issue. I admire people who fully embrace their age and where they are in life. I am trying to get there and wish everyone else will get there, too. May we all find peace with ourselves whatever age we are.