How common is "false pregnancy" in cockatoos?

Hello... My 36-yr. old umbrella cockatoo has a "false pregnancy" or "false egg" about twice a year. Each time, she swells like she has an egg, and picks her feathers, & stops eating, and nests (secludes herself). Each one lasts about 3 weeks, give-or-take, then she returns to normal size and behavior practically overnight. Each time, she adopts one of her toys to nest, and returns to normal a couple of days after we remove it from her.

She has a box on her cage that she sleeps in every night. We have been told the box throws her into nesting mode, however, she has had a box for 20 years, and is pretty routine in her "false pregnancies" with the season changes.

My questions are: how common is this? and when will she be too old to try to nest anymore?

Thank you!