I think it's fairly reasonable to say that it's way easier to get results in vision therapy if you're doing it as a child. I once asked this question to a vision therapist that I know and her answer was 'Children fix their vision without even trying.'
A big part of the reason is to do with neuroplasticity.
Neuroplasticity is significantly higher in children, although we have it for our entire lives. The different levels of neuroplasticity between adults and children is the reason why if you move to a new country when you're six, you'll learn to speak that new language perfectly, with no accent in a few years or less. But if you go to a new country when you're 20 years old you'll never get rid of the accent, even if you live there for 50 years.
It's the same reason why it's harder to get results when doing vision therapy as an adult.
The biological process of learning is different depending on your age. When you're learning a new language as a child, you build gray matter in the brain. When you're learning a new language as an adult, you lose gray matter in the brain. Sucks.
According to Anders Ericsson, becoming good at anything--whatever it is, playing golf, looking at things with both eyes, playing the piano--is achieved by leveraging neuroplasticity.
I'm starting to come to look at neuroplasticity a little differently these days. I think that maybe in order to achieve my vision therapy goals I have more than enough neuroplasticity. What is proving and proves to be my difficulty is knowing what to do, and being aware of what is happening and being aware of what my possibilities are at any given moment.
This may be a different way of looking at neuroplasticity. When you're old, you get set in your ways. Your excess synapses get pruned. This paves the way for quicker and more reliable high-level circuits, but it also reduces the number available options--ways of doing things, ways of thinking, habits, etc.
For a child to fix his vision, maybe the reason it's easy is because everything is new to him. It's not difficult for him for a new thought to occur, or for it to occur to him to do something in a different way. It doesn't matter as much to him whether he uses his eye this way or that way, because he has very little experience doing it either way.
That is why it's harder for adults to achieve one's goals in doing vision therapy as an adult. It's not that you don't have the required neuroplasticity. It's the habit of doing things the way you've always done them. The inertia which makes it so hard to probe around, and make a new connection.
This is why a good vision therapist is so important--and why even if you have a good vision therapist--you have to make a concerted effort to understand what you are doing, to remain open, probe around and think about what effect the exercises you are doing have on your visual system.
In other words, vision therapy is way easier for children because of their huge neuroplasticity and lack of preference for doing things in a certain way. The advantage that adults have is that, while they have significantly less neuroplasticity, they have more than enough, and they have the ability to intellectualize (if they have the temperament) how to leverage that neuroplasticity.