Fertility decreases with female age because eggs are not produced anymore after you are born. The amount of eggs used during a lifetime is different for everybody and some people can find that their reserve is finished earlier then others.

Determining your ovarian reserve can give you an idea of how much time you have left.
A special ultrasound called follicular antral count and a specific blood test called AMH can give you theses answers.

The ultrasound needs to be done at the beginning of your cycle (day2 or 3) while the blood test can be done at any stage.