It's doable, but it won't be a fast process.
Think, for a moment, about how and when humans learn to speak.
As kids, our bodies and brains are optimized to learn language. We pick up vocabulary at an amazing rate, and instinctively grasp not only words, but how they fit together, and how to produce the sounds and rhythm and intonation of the local dialect.
Learning a language as an adult is a whole other experience. We have to learn not only the new language, but how to learn a new language, because we've lost the neuroplasticity that made it so easy and natural. We don't pick up words nearly as quickly, and no matter how fluent you get, you may always have an accent. In rare cases where someone
never learned a first language in that important window, the challenges are even greater.
There's a similar developmental period in which we learn to walk, and birds learn to fly. If, for some reason, that window is missed, just like language, it can still be learned, but it will be much more difficult, and they may never achieve fluency.
They'll fly with an accent.
A bird learning to fly is learning more than just the mechanics of getting from point A to point B. They need extra strength to ascend and extra control to descend. They train their vision to see the world at high speed, and train their minds to react to what they're seeing. Flight is also an all-or-nothing sort of deal. Yes, they can hold a perch and flap to build muscle, but when they're in the air, there are no training wheels. They have to launch themselves, and a bird who has learned that launch = painful crash, isn't going to want to take that risk.
The biggest challenge, then, isn't one of mechanical ability, but of confidence and being willing to attempt flight, even when they know they might fail.
My TAG, Boo, came to me at 11 years old. She has full wings, but had never learned to fly and refused to try.
On the advice of someone from this forum, I set up her space so that her food was on a stand separate from the cage. (in our case, the stand sat on a plastic bin, with a 2nd plastic bin in between the food stand and the cage. To reach the food, she had to wall across both bins, including stepping across the dip where the two bins touched.
Once she was comfortable with this setup, I started to very slowly separate the bins. First just a centimeter, so she could still step across easily, but could see the floor below. Then a bit more, and a bit more. Each change, we left it as-is until she was confident and comfortable before moving on. I think it took about 4-6 months for her to do her first flapping jump to cross the space.
It's been a few years now. She's come a long way, and is now fairly comfortable with flying to and from the bins (we've done a variety of configurations as we've slowly increased the difficulty level - sometimes finding a new configuration that is the right balance of challenging but doable is tricky).
Her most recent achievement is managing to turn for the first time. Not a sharp turn, but even curving her flight path a little is something she never worked out before. The other thing I'm seeing for the first time, just recently, is the willingness to try, fail, and then try again. She has learned how to land without crashing, which makes all the difference, especially for a heavy-bodied bird like a grey, where crashes are more likely to result in pain and injury.
(Some people have success training their birds by gently tossing them onto a bed or other soft surface, so they can practice without risk of injury. I tried to get Boo to do that sort of thing, but she wasn't having it, and pushing would have destroyed her trust in me, so it wasn't an option in our particular case).
She still has so far to go, but the changes we've seen in her as she's learned to fly have been mental as much as physical. She's more confident and more playful and more mischievous, and we love it.
