I thought the reason that you "might" see more gifted children from older parents, esp. women who are older, is b/c you might have more time and patience when you are older, as compared to when you are young.
Some parents who are in their 30's or 40's and having kids may have more disposable income to lavish on the child as compared to if they are having a child at age 20. They may be more settled with their careers and ready to be involved with raising a child.
Obviously, you can be a young parent and have a gifted child!