OP - I currently have DS sleeping on my chest snuffling baby breathe on my neck. He is 3mo and I'll be 44 in April. He was 'kinda planned' in that we gave it a go but never ever thought we'd conceive. (It took me two years to conceive DD who was born when I was 40). First time we tried I got a positive result and immediately went into shock and had fears and worries about impact of a child with problems (Downs etc) on our DD.
We considered having the Harmony test done because it can give you a very early and definitive result for the tests the NHS will offer later, but we decided not to feeling we'd probably not want to terminate anyway. (It's very expensive but carries no risk to you or fetus - www.fetalmedicine.com/harmony-test).
My NHS tests came back giving me the same level of risk as a 20 year old.
I had SPD in my first PG and again in my second. But that's not age related. I found it worse in the first PG. However I did end up crippled with a bad back, probably because I moved house and carried heavy boxes when I was 7m PG with SPD!! Because of my anxiety and pain and the impact my bad back had on doing things with DD, I found it very difficult to bond with DS while PG and wondered right up until I went into hospital if I'd made mistake.
I was induced, at full term, because of severe back pain and DS was born in 2.5hours with no issues to me or him (DD was induced because I had a fibroid that they thought was impacting on her growth, and she ended up being an emergency forceps delivery. The same fibroid did not grow at a with DS.)
I fell in love instantly. We adore him and feel so bloody lucky to have had him at our ages. DH gazes at him with such love (which he didn't do with DD at the same age). He says it's because he knows what's coming and he has less fears, he feels freer to just love him in the same way we now do DD at her older age.
I envy you your older children who will most likely be besotted and willing to occasionally hold him for you while you nap, or get some laundry or cooking done. And you have baby sitters lined up
With DD at 3yo I am really in the thick of it (as a mother at any age would be) but I wouldn't change it for the world.
I kept asking midwives if I was 'really old' and they kept telling me stories of women much older. The person who had delivered before me was 47. If anything, I had less fear and scare stories about my age from medical staff the second time than I did the first - maybe because he was my second and they knew my body would know what to do, which certainly seemed to be the case.
I recommend you and your DH save every jot of leave to spend time with your baby when it first comes, over and above maternity and paternity. DH had about two months at home with us all and it made such a difference.
Good luck.