The impact of aging on female reproduction is well known. Overall, a woman’s age is one of the best predictors as to whether or not she will conceive with her own eggs.
After about age 43, the odds of this fall low enough (and miscarriage and chromosome abnormality rates raise high enough) that the likely best option is egg donation. After about age 35, when declines in natural fertility become sharper, many women are simply experiencing what we call age-related sub-fertility. That is, there is no specific fertility problem other than as we get older, the egg being ovulated may be less capable of sustaining a pregnancy.