I don't know, I don't have kids but nevertheless have had a truly enormous amount of family stress, both from my family of origin and my husband and his family. I almost think that if I'd had kids, I'd have been less available to be kicked around like the family football. And with kids, at least you have fun, too. The stress of the last 18 years definitely shows on me, all of it sodding family-related. IME, there are plenty of other stressors in life to age you outside kids. Maybe you and your friends are still quite young. I'm 51, and I find that no one gets a pass out of life stress. It'll find you eventually, somehow.
@Kasma "Not had kids and only myself to fawn over"? Yeah, not having kids really isn't like that. Not for me, anyway.
Gosh, lots of comments here about how women without kids look younger. It's really not that simple. There's a lot of loneliness if you don't have kids, and that's the equivalent of smoking quite a few cigarettes a day, I read recently. Plus your marriage is more likely to break up, according to stats, and for me, at least, I received a lot of disrespect for not having kids. And most of your friends are taken up with child-rearing a lot. There are far fewer people available for friendship, outings, evening drinks, etc. Yes, I'm free to do these things, but most people have vanished into a family unit.
All these things can be quite stressful!