During my last pregnancy, in the doctor's office I met a woman who was 51 years old, having what she grimly declared to be her last. She wasn't happy with it, but her age and weight had hid it too long so she had to go on and have this child.
At 33 I was considered to be almost too old to have kids, in other families but not mine. I had my first one 8 months after Mom had her last one. My second sister went to her first daughter's wedding while still nursing her last. And my first son was almost 17 when my last son was born. So age in this and the last generation isn't so important now a days.
As royals, they will have the best medical care in the world, so their ages aren't so very important yet.!! Not for probably the next 20 years!! But they will most likely be talked into having that next heir to the throne.