My son is four now, and I'm 39. We've pretty much decided that he'll be an only. The primary reason is financial, but honestly, I also love my little family the way it is. You've gotten a lot of great advice already, I just wanted to mention a few things I didn't see (on the basis of a quick skim; I could've missed something) below.
1. The "science" saying that it's somehow problematic for children to grow up as onlies, has been 100% discredited. There's no evidence, via any discipline (sociology, psych, anything) that being an only child is in any way problematic.
2. Having an only child challenges you to reach out to parents of other kids. This hasn't been the easiest thing in the world for me, honestly, since I tend to be very private and shy, but I really wanted my son to have close peer relationships, so I've pushed myself to reach out to parents of children the same age, plan playdates, invite other families to all sorts of things, etc. Again, doesn't come naturally. I feel like I step out of my comfort zone every day. But by the same token, it's been really, really good for me, and of course, even better for my son.
3. As we (parents of only children) age, we have an added responsibility to get our own ducks in a row. That means living will, planning for your own nursing care *before* the fact, etc. Again, it's a headache, but I feel good about. It feels like the right, responsible way to live my life.
I hope this helps. Good luck with everything!
Mira