I would say it is, but only because it's illegal. Personally I have had a supremely easy time scoring weed, but anything else is pretty difficult. No one I get weed from has acid for example (and it frustrates the hell out of me). Honestly the thing that really got me interested in drugs was reading my mom's Abnormal Psychology book at about age 11. Especially nowadays with more liberal attitudes towards weed, I see connection between weed and harder drugs weakening.
I think once a person decides they would like to see what it's like to be intoxicated they have the potential in them to try drugs. Weed happens to come first most of the time because it is the most common and readily available. Most of the time it's in high school and most of them aren't doing hardcore drugs.