I'm 39, 40 this summer and I'm pregnant with number 3. My first two are 22 months apart and yes, we meant to do that and we waited for a third because two close together was manageable (man on man defense my husband calls it) but three that close would have been tough~ for us, mostly DH. Of course we thought that the third would be about 4 or 5 years from DD but..Mother Nature is a little funny sometimes and it was a hard road getting here so it's going to me more like 6 1/2 yrs. If I had #3 at 38 there was room for discussion for a fourth, now, not so much, having babies over 40 worries me a bit.
We have friends that intended to have 4. Two close together then a few years later two more close together. Their third was a surprise, born 12 months after #2....so they're done. It was alot for them having three under the age of 5.
My step sister has 5 kids all within 2 years of each other...loves the chaos has never regretted a minute of it. LOVES IT!
As for your age, I've done nothing but freak out about it for the last 2 years, and my last obgyn was not supportive, "well, you are over 35 blah, blah, blah..." I've changed Docs and my new guy is great always reassuring me that I'm not old and that he and his wife had their last at 39...and that he'd rather take care of a pregnant, healthy 40 year old any day over a couch potato, junk food eating 20 year old. Did I mention I LOVE my new Doc?
There was another thread that had a link to a website called www.havingthreekids.com it's fantastic and worth a look.