The answer is "40s".
Fertility age differs from lady to lady. As age increases fertility diminishes. A lady is fertile at its young that is in 20's. Fertility diminishes in 30's.
In 40's lady is infertile. It is prudent to have pregnancy in your 20's or before 35 years old. Over 35yrs fertility abatements and pregnancy complexities likewise increase.
Infertility can be treated with different medications like IVF, IUI, blastocyst culture and so forth.