Hi Suraj,
Colony breeding is not the ideal way to breed cockatiels. If you are trying to colony breed, experts recommend that you get a group of young, unrelated cockatiels and allow them to grow up and mature together. When they are mature enough to breed, you need to have more next boxes set up than pairs of birds in order to minimize fighting over nests or raiding nests. While some breeders will allow their birds to start breeding at under a year old, for best results the male should be 18 months old and the female two years old. While they can breed at a younger age, there is a higher risk of failure due to emotional immaturity. Also, hens are at a higher risk of egg binding if they are too young, breed too often, or too old to breed. Even if all pairs are unrelated, as they should be, you have to find homes for the offspring because they may breed back to one of the parents or with each other, and you will begin getting weak birds with genetic flaws.
I don’t know if this older pair will ever accept the younger pair, but they are not likely to breed as long as both pairs are together. You can remove the nest box and let the younger birds become old enough to breed. The problem with this is the older pair has bred before, and if they are a few years old, for example, they will be getting close to being retired from breeding when the younger birds are finally old enough to begin breeding. If you leave them all together, do not give them any nest boxes until the young female is closer to two years old. And then give them 3 or 4 boxes.
Another option is remove the young birds to another cage and let the older pair breed. However, it is not likely you will be able to reintroduce the younger pair, even when they are old enough to breed.
You should do some research on colony breeding cockatiels. While most people do not breed this way, there are a few articles about it. But as far as this group goes, you may not ever have a breeding colony with them.
Thank you for asking Lafeber,
Brenda