there is a fertility window for a reason” they have hit the nail on the head!
The thing is the fertility window is a varied one, and for some women (probably not the majority to be fair) it does go up to late 40s.
Mt sister's running club buddy has recently fallen naturally and unexpectedly pregnant at 47. She couldn't face a termination. If you go on the pregnancy choices board here, every day sees a forty something woman shocked she's pregnant because society told her she wouldn't fall pregnant.
Where I'm from (border of NI/ROI) the influence of the churches on contraception meant many women had children til they could no longer have them, and that was generally mid 40s, sometimes a bit later.
When I was at school both my neighbour and school mate's mum's fell pregnant at 45 the boys are in their 20s, their mums, when I've seen them around town, look well (one looks way younger than her age). My great aunt had her DD at 45 - her DD (mid 40s) very close to and supportive to her mum (she also has a v good job and travels extensively, her mum is well enough and independent enough for that setup). In fact given her mum's relationships with her other children, I'd wonder who would be there to support her and be company for her if she hadn't had a "late" child.
What i would say that these are all natural pregnancies, and the older someone is (the further towards the maximum age for the "late" contingent in terms of fertility) the more dicey I suppose it is; when only a small minority of women would fall pregnant naturally at that age (say 46/47 upwards), and you're using younger, donor eggs to get pregnant .. I suppose you are pushing into "unnatural" (with all inherent implications) territory.
I suppose donor eggs do eliminate many of the risks (?) Father's age is also important however .. I think if the father is older, I honestly don't see why you wouldn't just go the whole hog and use donor sperm too