I'm 49, but at 43 it would have been a definite no from me, for the following reasons, aside from finances, concerns about my energy levels and whether we would have had enough room:
At that stage (my kids were 11 and 8, I was approaching a different stage (High School) and wouldn't have wanted to go back.
The baby would have been like an only child cause my others would have been so much older. I think the family dynamic would have been very difficult, for example finding activities and holidays to suit different age groups.
I would be at a different stage to everyone else. Most of my friends have kids of a similar age to mine.
TBH I don't think I would have felt equipped to go with a special needs child or a multiple pregnancy (more likely with age). Sorry if that offends anyone.
I would have had to go backwards workwise.
Elderly parents. My Dad was seriously ill whenn I was 43, again when I was 46 and again when I was 47, he died that time. I did a lot of going up and down the country to help look after him, he lived in the North West, I'm in South London. This wouldn't have been possible if the kids had been much younger (they were 15 and 12 at the time).
The impact on your existing children. Yes, they may love having a younger sibling, but it would have its downside.
Hormones. I started peri menopause when I was about 44/45. I've also recently had breast cancer, having to deal with surgery and multiple appointments would have been very hard with a small child who obviously wouldn't have understood.
By the way this is totally hypothetical cause it isn't something that happened to me.