When I was younger, I imagined myself with 2 kids (and a dog), but I've never actually wanted kids. DH would have gone along with the idea if I'd wanted a child (he has an adult son), but fully understands that I don't.
I really don't think that being a parent is all it's hyped up to be. It's exhausting (both mentaly and physically), loud, messy, expensive, relentless. Problems do not disappear when they turn 18 either (DH has bailed his 35 year-old son out several times recently - SS has mental health issues).
When I look around me I see a friend who basically entrapped a (rich) man by getting pregnant. She actively dislikes kids and farmed them out as much as she could. Another friend has completely destroyed her marriage in her quest to get pregant and her behaviour afterwards. She went through all the horrors of IVF and now she's had her child, her husband has basically been pushed out of the picture. A family member has 2 special needs kids (ASD), she is constantly stressed out, expecting things to go wrong, fighting for them to obtain support. Another family member has 3 kids, 1 with learning difficulties, 1 with ASD, 1 NT. She lives her whole life through them, in a "if you want to make me happy, make my kids happy" way. Another family member has the two worst-behaved kids I know (completely wild, swearing at teachers, etc.). My closest friends have one gifted child and one child with dyslexia, ADHD, ASD. Both kids are really demanding, for different reasons. None of these families are happy, as far as I can tell. They are, however, all stressed and exhausted.
Looking through everyone I know, I can only think of 2 truly happy families, where the kids are loved, nurtured and cared for and the parents seem to be enjoying life despite the occasional bump in the road.
People have listed numerous advantages, but it basically comes down to the fact that your time and money are yours (and your partner's, if you have one).
Having children so that they can care for you when you are older is 100% wrong and totally selfish. People who do this are probably setting themselves up for disappointment, in any case. Relationships break up, children move overseas, people sadly lose children...
I do like kids, I spend a lot of time with kids, but I'm happy that they're not mine and that I can give them back. I truly cannot see in what way having a child would improve my life. That, by definition, would be a bad thing for a child.
I'd say that anyone who isn't 100% behind wanting a child probably shouldn't have one.