My oldest were 23 and 20 when their half sibling was born. We raised two children and now are raising an only child, who became an uncle just after his 5th birthday.
We always intended to have more kids, but couldn't afford it at first. and when we could afford it, one of the now teenaged DD's was having psychological issues. we lived in the states, so that cost us money on top of time and effort and heartbreak. By the time DD and our finances recovered, I was nearly 39, and it took almost two years to get pregnant. We reassured ourselves that not getting pregnant was also fine: being double income with no children, being able to travel, to finally have nice things, to top up our retirement funds big time, all sounded good. We decided to try one year of fertility treatments as sold by our health care organization. I got pregnant the month we finally got fertility treatments approved, but before we attended the first appointment!
We hoped for a second, but moved to the UK (DH's home country) while I was pregnant. DS was born when I was 42, and I was too old for fertility assistance on the NHS and pregnancy just didn't happen this time. Within 5 years I had noticeable symptoms of beginning menopause, more and more erratic cycles, and after another 3 years, no cycles.
so, it's not necessarily too late, physically. On the other hand, I will not deny there are times I sort of regret not taking the other road, the one with more traveling, less frugality, and more retirement funds. Our life with DS has had more travel and less frugality than life with his older siblings did, but when he was diagnosed autistic, my ability to go back to work was delayed. I was finally looking for work in February...
unexpected things like autism and not being able to go back to work are the hard parts. having a baby, at last, with DH was wonderful. It is wonderful to see them together. Unlike you, a child, or more, was part of our original plan. It came up when we were dating: did I even want more children, because he did. If I didn't, then he didn't see the point of moving forward and getting to know my DD's (we had mostly seen each other when they were visiting their dad.). A dozen years later, he had accepted that we had maybe left it too late, and was beyond ecstatic when I peed on the stick that finally said "pregnant".
But it sounds like your DH might not want the experience you are currently brooding about. He accepted your DD and hasn't asked all this time for more. He may have seriously different expectations for the next 20 years than you are now developing. you already know how LOOOOONG 18 years is and how much can change. I still fear one of us won't be here when DS is 18. No matter what their situation, a lot of mid-30's age women get broody. Their partners rarely are, unless, like my partner, having a child was always the plan.
My youngest brother was only 39 when mom died, only 32 when dad died. He was a late baby, the rest of us were 10 to 17 when he was born. Like DH and I, mom and dad had moved when the rest of us were grown so he grew up alone. (They had moved 'home' to where they were born, so he did grow up amongst a herd of cousins and second cousins, the rest of us hardly knew our extended family) But, as a result, youngest DB was the only one of us near enough to deal with their illness and deaths directly. That was a lot of weight for such a young guy to carry. My DS will be much the same, except without the support of a large extended family of uncles, aunts and cousins on hand. My mothers parents lived to their 90s. I admit that played into my decision to try for a baby at 39/40/41. My now grown baby brother's presence played into it as well. At that point, it seemed things had worked out fine for my parents and him. My dad began having dementia the year my son was born and died the year he turned 6, my mother the year DS turned 12. Both were in their 70s. I suddenly feel much differently about my life expectancy. I know it wasn't reasonable to assume I would live as long as my grandparents, but now I am stressed I will only live as long as my parents, that I won't have another 20 years after DS is grown.
Looking back, I can't say for sure we made the right or wrong decision to try to get pregnant at that age. I love my son. I love having been able to be home with him, I wasn't with the DDs. I love having a child with a partner who wanted that child so much. OTOH, I have been a parent since 1984, and have at least another 5 years to go and I am so tired of being a parent to a child. You could have 20+ years of active child-free lifestyle before you retire. Your daughter should have 20+ years before she has to deal with elderly parents. your potential child might get 20 years too. Or maybe they won't.