When women get older is there is a point at which their bodies become more conducive to child bearing? PMS symptons change and so does their body composition. So certain women change biologically. And at a certain age the oestrogen levels become so inflated that instead of breast size going down after PMS monthly cycles they just stay enlarged and the same? Biologically women are created to have children by a certain age (generically.) So what other symptons can they expect if they are still not sexually active etc approaching their late thirties?

'at a certain age the oestrogen levels become so inflated that instead of breast size going down after PMS monthly cycles they just stay enlarged and the same?'

Women have decreased progesterone and prolactin as they age, and increased follicle-stimulating hormone (FSH) and luteinizing hormone (LH), with estrogen making a more dramatic drop after sometimes increasing. There are many variations of this, and they happen at widely different ages. Quirks in your hormone levels could be contributing to your enlarged breasts that don't go down, but you could also be hitting early menopause. You might have nothing 'wrong' at least past unusual hormone fluctuations.

(Breasts that stay large have nothing to do with a woman being more likely to get pregnant, in response to your first question. There's no specific age either, although the middle years of full menstruation are the most likely. You may be winding down by now, but there's no way to know without expensive tests.)

There are almost no generalizations that can be made for all women. The variety of all menstrual and PM symptoms is huge. There may be differences between women who have never born a child (nulligravida) vs those who have (primipara). There are differences around the world and in different cultures, levels of poverty/health/medical care, and so on.

HOWEVER, please read the following site about possible breast cancer or inflammation:
What Are The Causes Of A Swollen Breast? | LIVESTRONG.COM
And get a mammogram if you haven't had one recently.