Both males and females should be at least 1 year old before being bred.Some breeders even recommend waiting until at least 15 months.
They are able to breed younger than this but are not emotionally ready for the stress of raising chicks. A young hen may look like she is in breeding condition with a very dark brown, flaky cere. This is an indication that she is growing, not a sign that she is ready to raise a clutch.
A hen that is ready to breed should be over a year old and under 4 years. She should be in good physical condition (not under or over weight) and should have a dark chocolate brown cere.
A cock in breeding condition will be in good physical condition, over 1 year old and have a deep blue cere (with the exception of those mutations in which males have a pink cere-albinos, lutinos, and recessive pieds). Click here to see an example: Examples
If either gender bird is moulting it would be wise to remove the nest box as moulting and raising chicks would take too much out of a budgie.
Breeding your budgies too early can result in failed clutches such as infertile eggs, eggs not being properly sat on, breaking of eggs, egg bound (which can kill your hen) and more. Be patience and wait to the appropriate age.