Age and fertility are a very close correlation.

In my infancy years as an egg donor agent, I learnt that the first questions a fertility clinic asked is how old the female patient is.  Usually post 38, donor eggs are recommended. 

Unfortunately for woman, fertility deteriorates with age quicker than for men.  Although woman may still have a ovarian reserve from 38, the quality of these eggs will start to diminish.  These causes are life, hormones in food, stress, pollution, exposure to disease, radiation, autoimmune disease, other medical reasons and of course unknown factors.  Older eggs are also at a higher risk of chromosomal deformities.  So whilst woman in advanced years may elect to try conception with their own eggs if they are still ovulating, they are in a higher risk category and are commonly encouraged to consider donor eggs.

According to the Society for South African Society for Reproductive Medicine (SASREG), there is a medical cut off of 34 for egg donors.  Woman may remain very fertile after these years, but cannot be accepted as egg donors.  The IVF process with a third party costs alot - financially and emotionally.  All efforts need to be made to give recipients the very best chance of conception and thus the medical criteria about age.  Having said that, a 21 year old is not necessarily more fertile than the 34 year old, but this the medically accepted age group for woman to sign up as egg donors, between 21 and 34.  Applications are also received between the ages 18 - 21.