What is a likely explanation for the results from cross-sectional studies showing that older people are less likely to score as well as younger subjects on traditional intelligence tests?

cannabis

explanation:

(assuming this is the us) google it. law states you can't have weed.

explanation:

allergens: overstimulate the nerve system

neurotics: attack nerve cells

mutagens: damage or otherwise alter dna sequence

generators: cause defects during growth and development  

carcinogens: cause unchecked cell growth

i hope you get the idea according to me these are the effects that happened.