When wishing to get started as a breeder of cockatiels (or any other birds, for that matter); many people are often unsure of the best way to get started. If you get extremely young birds, it takes a LONG time for them to be breeding age; so is it really worth the wait? In my opinion, YES...and VERY wise, in most cases, anyway.
What about those ads for "Proven Breeders" that you see in almost all bird-related magazines? Why not take a shortcut and go that route? The main reason is quite simple: MOST people are not going to be offering any of their best breeder birds for sale. On rare occasions, there will be someone that for compelling reasons is leaving aviculture, and thus divesting himself/herself of their flock. 99% of time in those cases, if their birds are THAT good, friends will have picked their best birds already and bought them. So, are people being dishonest when they say they are offering proven breeders for sale? Not always; but perhaps not being TOTALY honest either. That proven bird or pair may lay infertile eggs...or not incubate their eggs...or eat their eggs...or peck their young...or simply not feed their young! Yes, I do believe there are circumstances in which it is safe to buy "proven" birds; but I would limit this to following: (1) If you're PERSONALLY familiar with the birds and (2)if you know the person you're to buy from and trust them completely.
Then there are factors to think about such as costs of older birds and possibility of long term exposure to disease....In over 23 years of raising birds, including 7 with cockatiels; I'm convinced young UNRELATED pairs are the best bet in most cases. Beware of shortcuts!