Assuming the older parents WANT and could AFFORD the children, it becomes an issue with biology. Fertility declines, especially in women, and that declining fertility makes it harder to conceive a child. That means it may become longer between pregnancies and at some point, pregnancy becomes impossible because menopause happens. At that point, the woman has stopped producing eggs which means pregnancy is impossible.
Most parents stop having children at a younger age either because they have the number they can afford, they have the number that they want, or they had a certain age that they intended to stop having more at. For example, my husband and I had two of those goals. We wanted two children AND we wanted to have those two children BEFORE the age of 30. We did NOT want to be caring for children under the age of 18 by the time we reached the age of 50. That was our personal choice and we both agreed.