First, consider which kind of adderall you're taking. I think there's like, 3 kinds, and each one lasts a certain amount of time.  Extended releases can last from 8-12 hours.  Regular adderall is supposed to last 4 hours or so, but effects might be perceived for longer periods, especially because of drop off effects or rebound/post drug tiredness.  Height and weight definitely dont make a difference.  This drug is not affected by the rest of your body.  As far as age goes, i think the chemistry and action of the drug is just the same, but as far as "feel" and effects, it really varies with each individual.   Haha, having ADD, i've gone overboard on researching the pharmacology and biochemistry of all this stuff. 

I take 30mg 3xday of adderall - typically it lasts 3 1/2 to 4 hours

Does anyone know more infor about the "8 hour one" perhaps I shouldnt post this but before I was diagnosed I took an adderall my first of college and I remember specifically it lasted 8 hours b/c I remember always saying to myself wow that was the best 8 hours I accomplished so much...

I just didnt know if there was a seperate 8 hour one - I dont like the XR.

Now sometimes when I take my adderall it varies like if I take it at 8am I will fell like I need to take it like it is wearing off around 10:30 or 11 which is before the typical noon dose...does anyone else experience this or have any recommendations?

Most books commonly cite that Adderall should last anywhere b/w 4-8 hours.  I do better taking the tablet than the capsule (XR - slow release).  But, I think the longevity of the dose varies person to person.  For me, any dose of Adderall I take (20-60mgs) lasts up to 2 hours.  Not great........

Good luck.  Don't expect a quick fix (I did).  It often takes months to find the right med (or combo of meds) and dosage for you.