I receive a lot of emails on how to differentiate the sex and age of budgerigars so I thought I'd post a brief note on this topic.
To those who have my book, the following is a virtual extract from Page 37 and 38.

The color of the cere (membrane above the beak) is the key to determining whether a budgerigar is a male (cock) or female (hen). At about 3 months of age, the adult color of the cere shows blue for cocks and brown for hens.
AT A VERY YOUNG AGE (e.g. four weeks old), you should be able to determine the sex of the bird by paying careful attention to the following:

On a baby cock, the cere is a uniform light purplish color, which will turn blue sometime after three months.

On a baby hen, if you pay close attention, you should be able to distinguish that the lower part of the cere is a light bluish haze while the upper part especially around the nostrils is a whitish color. This will turn brown sometime after three months.

In adult budgerigars you will notice a white ring around the perimeter of the eye whereas in baby birds (chicks) the eyes are pitch black with no ring.
ALSO, most chicks (WITH THE EXCEPTION OF YELLOWS, ALBINOS AND SOME PIEDS), will have bars (stripes) on the head that continue right down to the cere (unbroken caps).
These bars begin to recede at about three months and will eventually disappear (cap broken). It is about this time that the cere color becomes distinct, either blue (cock) or brown (hen).

Gil Solomon
Budgie Care Publications