It’s nice to have some science to back up common sense. From Older Boys Really Are a Bad Influence:
Parents who forbid their daughters to date older boys may be on the right track. A study published on Thursday finds that teenage girls who associate with older boys are more likely to smoke, drink and use drugs.
I was quite surprised to find correlation not confused with causation:
The survey of 1,000 teens found that friends do influence behavior, or at least reflect behavior, The National Center on Addiction and Substance Abuse at Columbia University said.
Here are the stats:
The study found that 58 percent of girls who had boyfriends two years or more older drank alcohol, compared to 25 percent of the girls who dated boys their own age or not at all.Fifty percent of the girls who went for older boys or men smoked marijuana, compared to 8 percent of the other girls, and 65 percent of these girls who preferred to date someone older than themselves smoked, compared to 14 percent girls who stuck to younger boys.