Your other child would not benefit in any way.
Please don’t drag them into this as this is solely about you and what you want.
I don’t think the age is too old, although I think having a 60yo mum whilst you’re still at school is a little unfair and I couldn’t imagine never being able to work or have hobbies without worrying about childcare etc.
If you’d accidentally fell pregnant then it would be different but I think you are wanting a baby for the wrong reasons and so for that reason I think YABU to even consider it.
You are likely feeling lost because you have no sense of self because you have no relationship, you’re approaching middle age your other child is starting to need you less.
This is like the sports car equivalent to a man’s midlife crisis lol.
Gently, your eldest has had a shit childhood and instead of trying to start again and doing it right this time around, then you should be focusing on making the rest of their childhood good and focusing on making yourself happy without the need for another person (partner or baby).