My Green Cheek Conure, Bowie, has been using his Aviator Harness since I brought him home at 11 weeks old. It took a good month to get it on him without any fuss at all, but then it took another couple of months of just having him wear it and taking him out in it for him to finally ignore it and not chew on it constantly. Since then he doesn't even remember that he has it on...
This makes me want to try again. Boomer was a little over a year old when I first introduced the harness to him. He did not like it so I went the route of birdie backpacks for outdoor treks. I wonder if, now at 6 years of age, it would be very hard to harness train him again?
I'd definitely give it try BoomBoom. You have to remember that when it comes to them wearing a harness, it's one of the few things that birds overwhelming hate as a whole, regardless of their age, unless they were introduced to the harness/flight-suit while they were still being hand-fed and un-weaned. Thus, getting a bird to learn to put on and then accept a harness can be done at any age, that doesn't make any difference at all.
I currently have 4 birds that I got in the last 4 years, 1 bird each year basically. And each one I got from a private breeder who hand-fed/hand-raised them from 2-3 weeks old. I got my Cockatiel 4 years ago when she was 8 weeks old, then my Quaker 3 years ago when she was 12 weeks old, then my Green Cheek 2 years ago when he was 12 weeks, and finally my Senegal a little over a year ago, when he was 13 weeks old. Every one of them HATED the harness! They threw fits, they screamed at me, they ran away, etc. So with each one of them I simply did it the "right" way, aka the "slow" way, lol. And each time it worked...
Each bird is going to be different, but on-average with my 4 birds, it took them between 1-2 months of working with them a bit every day to get them to put the harness on without any fuss or issues. And then it took another 1-2 months to get them to forget about the harness while they had it on and to stop constantly chewing on it.
My Green Cheek and my Senegal were by-far the easiest birds to harness-train, as they both took about a month to get through each "phase" of the process. Ironically, my Quaker pretty-much allowed me to put the Aviator Harness on her immediately without any issue at all right from the start, she never made a sound, never bit/nipped, never wiggled or pulled her feet/legs out, etc. I literally just put the harness on her after it had been hanging inside of her cage for a week or so. HOWEVER, she was doubly bad at accepting the harness being on her, she just chewed and chewed and pulled and ripped and wrestled with it constantly. It took her a good 2 months if not longer to finally give it up, lol. My Cockatiel hates the harness but has no problem putting it on or wearing it at all. However, I don't usually put it on her as #1 she can't ever fly again due to a wing injury, so I can just take her out on my shoulder, and #2 the few times I did take her out with the harness on she just looked so miserable. It was sad. She was like "Mamma I'll wear this thing if you want me to, but I refuse to have fun while it's on me"...
So it just depends on the bird...The harness is one of the few things with regards to birds that is pretty-much universally a process that is going to take a minimum of a month or two, and can take some up to a year or more.
The best thing I can tell you is that, at least in my own opinion, the reward for not only your bird but for yourself once you can take them out with you anywhere on your shoulder is well worth it...It really is. It's so nice to walk around with your bird on your shoulder, showing them off and having them have such a good time seeing new things every time you take them out. It's just not the same as having them inside of a carrier or a Pak-O-Bird type thing.