Yes females can be fertile up until about 12ish years from what I've read, males into their teens if taken care of properly. It all depends on how they were taken care of though.

 

Cockatiels can remain fertile most of their life. With hens it is the egg count that goes down, and with males they can be partial infertility in a clutch.

But there are some things that can effect fertility. Regular uses of antibiotics such as preventative treatments can cause short term infertility, and if used extensively permanant damage. Selection of a good fresh quality of seed and veggies helps. If there are any pesticides on the veggies/greens they can contribute to infertility.