After the age of 35, there is a predictable decrease in fertility potential in most couples.

A woman’s reproductive potential begins shortly after puberty at age 13 and technically continues until as long as she is ovulating and having periods until the time of menopause (which is on average of age 52).

The likelihood of pregnancy in most couples is directly related to egg quality and egg quality will decrease with age because there will be an increase number of egg cells that are not genetically perfect as well as decrease in the number of egg cells that are present.

More specifically, the likelihood of achieving pregnancy relate to age:

Is rather constant from puberty until the age of 30
Declines further between the age of 30 and 35
Declines more rapidly after 35 and 40
Declines even more significantly after the age of 40 and 44