However, it's very humane to bring to other people's attention that their prospective children might not be healthy.
No.
> and suddenly leave you grieving in a way you'd never be
And we should consider that when deciding to enter a relationship.
35 is roughly the cutoff for "everything's going to be fine without much effort or thinking", so plan ahead.
That's when you're going to start getting help from fertility doctors, consider IVF, get test results showing that your own sperm's motility is not perfect (i.e. you've got a clock too), higher chances of Down syndrome, etc.
For some folks that's not a lot of time to get married, buy a house, a car, get a stable job and bring your financial situation in order. These aren't strict requirements, but the amount of stress involved when one of them is missing is something you'll have to handle, so at the very least you and your partner shouldn't be completely oblivious of the soft deadline at 35.
The note from your daughter, while sweet, can also come from those you help. For example, sponsoring a childrens school in India, or mentoring an orphan through a Big Brother program.
I think the main point here is you are in service by helping others, instead of focusing on yourself.
I imagine it’d be incredibly fulfilling to know that you helped coach a troubled youth through a turbulent period to land in a good place.
The other day I was flicking through my notepad in my home office and found hand written notes from my 6 year old daughter saying how much she loved me. Yes kids are a massive responsibility, but the payback you get if you do it right is immeasurable.
I'm not sure you'd get quite the same from the things you listed, perhaps a little...?
Anyway what is suggest is you enjoy your 20s, but make sure you are in a position to have kids by your early 30s... Otherwise it may be too late for your other half (women's clocks do tick rather fast). If you wait until she is 40 then you are really risking it.