I have a 34 and a 24 year old cockatiel. I think genetics, mutation and gender also play a big part. Every senior tiel that I have heard of, including mine, are males and either normal greys or pieds. Those mutations just seem to be hardier from what I have observed. And males I think because egg laying takes its toll on females. Just my observations, not based on any real studies or anything like that.
I am not the original owner of either bird. So I can't say what they ate previously. Since cockatiels are granivores, I believe they need seeds/grains in their diet. I was taught that by a bunch of veteran cockatiel owners from a cockatiel forum I used to belong to. I know that's contrary to what is said about a lot of other species. So my tiels get a combination of seeds, senior nutriberries, fresh vegetables and freeze dried vegetables every day. Periodically Higgins Worldly Cuisine and once in awhile a little bit of nuts as a treat.
My 34 year old's original owner gave him petamine. No idea why because it's a breeding supplement. And he's a boy. But since he had been on it for 29ish years I wasn't going to stop. So he also gets that and I give him Healthy Bird which is an anti-inflammatory supplement.
