I'm 48 and I have 14 yr old twins, both with significant SEN. DS is still in nappies and hasn't progressed much beyond a 7/8 yr old child.
For a very long time I yearned for a third child, primarily as my partner fucked off when I was pregnant and I had to go through it alone. I wanted another child as I loved being a mum, but I also really wanted the experience of having a baby as a couple.
I've been with DP for 13 years now (knew him previously for a long time as a friend). We always planned to have another but eventually, we had to concede that it would be a bad idea due to the needs of my DC. It just wouldn't be feasible to support my DC the way that they need, and give enough love and attention to a new baby/toddler etc.
Plus there would obviously be the risk of having a third child with SEN, with potentially even greater difficulties. DD struggles but DS really has it tough, he's never going to live independently so having a third child with similar needs could push us to breaking point.
Occasionally I dream about being pregnant and having a baby again. I get wistful when I see my DC's tiny clothes or old toys. But I think those pangs are more about wishing they were tiny again rather than actually wanting another child.
For a very, very long time I struggled with not having another baby. I think I've finally come to terms with that and I wouldn't change it, even if I could.