Do you have any citations for this? My personal experience was totally the opposite. I could always get an over 21 friend to run to the liquor store and get me whatever I wanted. Scoring weed was texting a friend of a friend and hoping for the best.
Alcohol is a more damaging drug.
Making less harmful drugs illegal, and the more harmful drugs legal is backwards.
Ideally, no children would have access to recreational drugs under any circumstances. However, we don't live in that world. We live in the real world.
Young people will find it easier to find a weed dealer their own age than one that's 21+ AND willing to do something illegal for them (provide alcohol to a minor).