I'm only 26, so I can't really help you on the having kids after a certain age part, but I might be able to help with some other parts. As a mother of three, and a family on a tight military budget, I can see where you are coming from. My husband and I had our kids YOUNG, and all three within a year of each other(twins LOL). Those three are so very very close, and now I want another child so badly, but I am worried about the children being so far apart, and also financially. Add to that that my husband has severe issues from being a part of a war that he wants no part of, and seeing one of his best friends killed right next to him, and you have somewhat of a situation. My husband and I are just now starting to get closer after 2 years of fights and frustrations because my husband refused to seek medical treatment for his issues, and I pushed him to seek help, but he would totally ignore me. But he is finally getting help, so now it's a non-issue.
So at this moment, we desperately want another child, and we have some serious complications. I tried to shrug it off, and pass it off as "baby fever", but every time I look at a mother holding her baby, every time I work in the nursery, my heart clenches. This is not a desire that will go away. So... I am sitting down, and calculating our finances, times, issues, and this is the bottom line. I love my life, I love my children, I love my husband. I would love any child that came along with all of my heart, and not hold back. So here is what we have decided. If God wills us to have another child, it will happen. We will try, but not get stressed out about it, not bother with tracking when I am ovulating or what have you. Just enjoy ourselves, and God willing, we will have another child, if not, that's OK with me too. We have a family that many would envy, and I'm ok with what we have now, we have a great life. But on the other hand, I would welcome another child with wide-open arms. I hope this helps you out some.
P.S. I just remembered that my principal's wife in Jr High had 2 more pregnancies at 39 and 40. She had healthy pregnancies for the most part, she was put on bed rest for 3 months with the first one, but the other one went off without a hitch. And she had 2 very healthy babies.