20 years?

[Brighty009]

Just wondering, what will happen when my horses reach 20 years? Do they keep aging or stop aging or die?

[Questhaven]

They age until they die. pregnancy seems to help keep mares live longer so if you have a mare you don't want to lose breed her. I think my oldest was 41.

[Brighty009]

on average do they die at? 30-ish?

[BlackOak2]

on average do they die at? 30-ish?

I believe the countdown starts at 17 years (at least that's the earliest any of mine have died). After 17 years, it's a climbing percentage chance to die as each turn is used.
Keeping them pregnant, negates death until the day (turn) they give birth. Be aware that they can die on the day they give birth.
It seems most of mine disappear between 20 and 23 years.

[Brighty009]

The mares die after giving birth? How do the foals survive without mare's milk?

[BlackOak2]

The mares die after giving birth? How do the foals survive without mare's milk?

The mare can die after giving birth. She may not; that will allow you to breed her again and keep her alive for longer if that's what you want.

The foal will survive automatically without mare milk. Think of it as supplemental mare milk or a nanny mare. You do not even need a mare on your farm and you can purchase a newborn foal without consequence.
That may change in the future, or it may not.