I doubt that. How many people try cigarettes for the first time the day they turn 18 or alcohol at 21?
Anecdotally at least, people first try these substances when they want to, not when it's legal for them to do so. I can't imagine this being significantly different for marijuana - it's already so easy to obtain as an illegal substance.
Anecdotally, alcohol and cigarettes are substantially more widely exposed to minors than marijauna is. That will likely change when everyone's older brother can pick up a bag at the store.
Anecdotally at least, people first try these substances when they want to, not when it's legal for them to do so. I can't imagine this being significantly different for marijuana - it's already so easy to obtain as an illegal substance.