Here in the US, the 21 age drinking age is a direct result of lobbying by an organization called MADD (Mothers Against Drunk Driving). This was back in the 80's when there was less regulation about DUIs. I personally think it's horse shit since it targets only young drivers when the problem is not exclusive to them. The law was also supported by these mothers because it forces now 18 year old adults to still be dependent.

The law influences a lot more than just your ability to purchase alcohol. Liability plays a huge issue. For example, most hotels will refuse to give a room to anyone under 21. They don't even want to take a chance that there is underage drinking done in one of their rooms, so they just refuse to allow under 21 patrons in unless they're with someone over the age of 21. That over 21 person than assumes all the liability for the under 21 customers.

Liability is a financial mess. I don't have parents, so I was responsible for purchasing my own insurance. My rates on everything from auto to renters was absurd compared to what I pay now at 24. Even with good credit and nothing but my age to hold me back, my rates resembled someone with poor credit and a criminal record. It also didn't help that I'm a male.

Lastly, many places prohibit any one under 21. For example, you can't even sit at a bar even if you don't order alcohol. Many places won't even allow entry if alcohol is present like bars, clubs, casinos, events etc... (Fake IDs have become the norm). Then there is just plain discrimination like some work places and rental communities refusing to accept people under 21. Nobody wants to hold the burden of liability that the law creates.

For some reason, you can be a bartender while still being 18.