Comment by JohnBooty

    One of my assumptions is that it also has a lot 
    to do with the humans metabolism that changes 
    over the years. 

That theory makes sense to me. However, my anecdotal experience is that I never tried any of these meds until my early 30's.

The first few weeks on Adderall were f'ing magical. I remember tearing up, I was so happy. Finally I was laser-focused. A bit emotional thinking of it now.

The magic faded fast. Adderall was still a large net positive for me for ~6-7 years. But never like those first few weeks. I guess I got somewhat close to that original experience a few times, after extended breaks from Adderall and restarting it.

Anyway, this is of course a sample size of 1. But since all the experiences described above took place in my 30s I don't think that metabolism change was a major factor. Certainly not within the first year. Again I know... n=1 =)