It's very individual and what seems normal depends on where you live.
People used to have larger families and no contraception, so fourth, fifth and more children in your 40s wasn't uncommon, first ones were. That's what's changed - but this wouldn't be your first.
I had my first dd a month before my 39th birthday. It felt just the same as being 33 to me. I was fit and healthy and I don't find lack of sleep any harder now than when younger, if anything, years of practice has diminished its power to surprise.
I think now at 42 and hoping for dc2 the biggest difference is I'm not as fit, because of less free time, rather than feeling 'old'. Longer term, I feel a duty to stay fit and healthy as long as possible, both to enjoy time with my DCs (and any GCs!) and to avoid being a burden.
One thought I have quite strongly is that, for dd, being the only child of older parents could be lonely and burdensome, us entering decrepitude just as she's busy with her own DCs. I do feel a sibling could help lighten that load and leave her less alone in the family.
But, the risks of older pregnancy loom large - I really don't want to create an addititional burden for her, by adding a disabled sibling to her adult load. Of course some of those outcomes depend on your attitude to testing and decisions following.
That's definitely a reason to try sooner than later, now.