It also seems rather strange to take one property of their lives and use it to explain their in your eyes loss of abilities. Not doing something for a long time also has this effect, so perhaps they for many years didnt need to do calculations in their head, or use their high school french and so on.
If you said you knew a twin, and one of them smoked weed daily and they had identical lives, and one of them lost cognitive abilities and the other didnt, i would be more likely to give your reply any credit.