very good question I've thought about a lot. https://www.healthcentral.com/article/adderall-premature-aging-skin
"According to a report on Erowid, an organization that provides non-judgmental information about psychoactive plants and chemicals, amphetamines can cause an increase in oxidative stress in the blood and tissues and, at higher doses and frequencies of use, it can cause premature aging. Stimulants, such as Adderall, can also cause dry skin, which can make your skin appear worn out and aged. Another possible reason your skin is not looking as healthy as before is because Adderall can reduce your appetite, which can lead to not getting enough of the proper nutrients."