HisAngel, I've had birds taken to pellets just fine even though they were older birds!
For a long time, my flock wouldn't touch pellets, and since none of them ate pellets, I didn't really bother with them. I did have one bird go nuts eating pellets for a couple of moments, until he realized what he was eating, then ignored them after that.
Then I got Charlie. Since he was eating pellets, I ended up retrying with the rest of the flock. I don't think Charlie ate pellets for the first 10 or so years of his life, but I could be wrong. At the time, I had another conure named Noel, and I would offer him pellets, but he didn't seem too keen on the idea. That is, until I started noticing that the pellets were being eaten! I thought I had some success! Well, no, not really! The budgies were going into Noel's cage and eating Noel's pellets, even though they were refusing pellets in their own cage!
As the saying goes, the grass is always greener on the other side!
And so with that, I started getting my flock to eat more pellets.
The tiels and budgies didn't seem to be huge pellet lovers, not until I tried the Vetafarm method (which is in the link provided above! crushed pellets mixed with seeds with some water added). They started eating pellets a lot better since then.
I find it easier to get birds eating pellets if you already have a bird(s) that eats pellets. Faye, one of my tiels, took to pellets as if she had always been eating them. I have no clue how old Faye is and she has had at least 3 homes prior to mine and I would not be surprised if all three homes only fed her seeds. I've had her for 6 years now, she she has to at least be in her teens, if not twenties, I presume.
Jayde was 3 1/2 when I got her, and I was told she doesn't eat pellets. She does eat Nutriberries, but not pellets. Funny thing is, she's been eating pellets from day 1 of being here! I didn't even try any conversion methods with her, she started eating the pellets on her own! She did have both Nutriberries and pellets available to her and she definitely didn't seem to enjoy the pellets nearly as much as the seeds! It's been 8 months now, and although that still holds true, she does eat pellets better!
I have predominantly smaller birds in my flock and the majority of them are rehomes of various ages. All of them will eat pellets, even though the majority of them came to me eating seeds! Jayde is the youngest member of my flock being about 4 years old, and the rest I assume are ~13 years old and older. I don't know who the oldest member of the flock is, but I do know that my bourke parakeet is 17-19 years old and my mitred conure is or will be 20 this year. Considering most of the tiels are ages unknown, they could be in their mid/late teens or twenties.
