These messages are lovely - as an "ancient" mum myself I have to say that my actual age never came into it... if you are healthy and happy, financially stable and want a baby, I think you should just go for it.
I had my first at 41 and am due to have my second at 43 in a month's time - no-one caring for me seems at all bothered by my age. I read recently that the average age for a last pregnancy used to be 42 before the 60's so it was no big deal then either.
I do think about having "less time" with my children, but then my mum died when I was 30 and she was 57, so there's no predicting it. I have taken care with ds1's memory box, and I have a book by the bed which I write a sort of love letter in about once a month so that if I do pop off early she can be sure that we loved her dearly - and with a sibling on the way she won't be totally alone.
One big and amazing benefit is that husband who is a few years older recently retired, so he is a hands on dad - he had fun telling people who asked whether he had a project for his retirement that he was about to be a dad!!!
I don't think I'm more tired than I would have been when younger - what I am, is less resentful! I went into motherhood after 20 years of saying I didn't want kids, (with a list of their "disadvantages" at the ready) so I can hardly blame the little sweetheart now! And actually, the joys and little pleasures of being a mum have been much more satisfying than I ever imagined - but then she is only 2 so we have interesting times ahead!
I would not let an arbitrary milestone such as being over 40 stand between you and another child - there are much more important considerations.
So good luck OP - it was three days ago I think you posted this - so you may already have started the journey!!! Fingers crossed for you!