That is a good question and truthfully, I don't know the answer to it. I have 8 Budgies, and the 4th one is a Lutino. I have had her 9 months and so she is probably at least 10-11 months. I bought her at the store and she was probably one to two months old, then.
I have noticed in the past week, that Scooter's cere is not just beige anymore, but is a crusty brownish/beige. From what I am told, that means she is at the mating stage. I am not sure what age one considers a Budgie an adult, but I am GUESSING at least a year old. From what I've read, it is recommended that females should not be bred until they are at least a year and maybe just a little older.
I noticed that A-n-M replied to your thread but did not give any details. Hopefully he will come back and fill us in - or maybe someone else will come along and help. I wish I could give you a definite answer but I can't. Sorry!