I had a baby young because I don't want to be an old Grandma.
When my kids are grown, I'll still be young enough to live my life.
It makes me laugh. For one, once you have kids, your life is over. Just because they turn 18 doesn't mean you stop being a parent. Yea, you probably won't have a lot of the responsibilities you did when they were 6-7, but you're still their parent. Do you really want to be 40 wearing glittery eye shadow trying to reclaim your youth at the club with a bunch of kids young enough to be your kids? Believe me, not a good look. It makes you look desperate and on the verge of a midlife crisis.
No, 40 is not old, its a womans prime. Believe me, I'm well past 40. But believe me, if your interest is still that of a 20 year old, you need therapy.