Statistically, it is a little safer to have kids before 30, and a little more safe to have them before 35; your body is ready to have kids at *thirteen*, so ten years after that (around age 23), your body is primed to go.
With so many women waiting to get married until they graduate college, and waiting to have kids until they get their big job or until they (and/or their spouse) finish grad school, more people are 25-28 before they're ready to have kids. That gives you a smaller window to work with. Not like one of my ancestors, who we found in the 1900 census with 8 kids still living out of 11 pregnancies, and who had kids at age 18 through age 40!
She may want to spend more time with them as a young mom (easier to stand up to your teenager at forty than at sixty!). Or maybe there's some medical weirdness in her family that means it'll be hard for her to get pregnant or maintain a pregnancy. Or she just may want more control over her life.
