Hi,
You need to separate these birds and take the nest box away. Both birds are too young to be breeding or paired with another. Cockatiels should be at least 2 years old before pairing them with another and letting them breed. In the wild they would never breed at this young of an age. Serious cockatiel breeders often wait for their birds to be 3 years old before pairing them up. Young males are more interested in mating than caring for a mate, eggs or chicks. He will tend to form bad habits like chasing the female so she won’t nest, breaking or eating the eggs or killing the chicks, all so he can mate again. A female needs more time to fully develop so she can safely lay eggs. A young hen is at a higher risk of becoming egg bound and dying. You need to keep them separate until the male is at least 2 years old. Otherwise if he forms bad habits, it will ruin him for ever being a good breeder. Chasing the female out of the nest box is already a bad habit.
Thank you for asking Lafeber,
Brenda