Why would you want to be 40+ years older than your kid?
I would not want a mother who is grandma age unless there were already several kids ahead of me. I can't imagine the relationship a child would have with their mother that old. What can you do together? Child so active and crazy and mom is falling asleep on the couch and needs to take breaks. Unless mom is one of those rare women that took care of their health, aren't overweight and are still very active. But most women around this age are overweight and their idea of enjoying life is watching TV while kids play video games.
When you're 20, your mother might be dead or in a nursing home. Usually kids want their mothers around as long as possible. Sure, your mother could die when you're 10..but still. That's kind of different.