Hey, man. I think that people are going to have different results based on their technique and how hard they work on it. It sounds like you have a good head start that most people wouldn't have. I believe that nearly everyone can make progress if they really try.

Proper oral posture is certainly not going to hurt you in the long run so you may as well bloody do it starting now.

That's what mewing is about, creating more space in the mouth by pushing your tongue against your palate. And as a concequence widening the jaw among other benefits relating health and beauty. Just make sure to perfect your mewing technique and be patience

In a few months I have seen positive change. Minor but in the right direction. 

See success stories which is stickied. Even adults in their 50s.

Factors that may contribute :

1. How frequently your tongue can stay on the roof of your mouth (people who talk for a living can't see as quick progress due to positioning being less constant)

2. How narrow your palate is to begin with. Narrower palates will have a harder time keeping their tongue in place.

3. How far back your maxilla is to start.

4. How good your technique is. Proper posture is lost on many, so the learning curve can be steep. 

Results aren't typical as the disclaimer goes. But they are possible .

It's all about hard work.