Although cockatiels are capable of reproducing by the time they are a year old, they should be prevented from doing so until they are at least two years old. This is especially important for female cockatiels: Laying eggs requires much calcium, and when a hen lays eggs at too young an age, the egg-laying can interfere with the calcium required by her bones.
To learn more:
A great place to learn more about Cockatiels is Tiel Talk, a Cockatiel bulletin board where tiel owners share tips on raising, taming, breeding, and caring for their fids ("feathered kids").
You can see pictures of Cockatiels at numerous sites, including bittybirds.com.