In the United States, the legal drinking age is 21, not 18. This was standardized in 1984. Before that, some states had lower minimum ages. In most (but not all states) the law is about purchasing alcohol, not consuming it. And there are always religious exceptions.
It is indeed illegal in civil law to ask an adult to buy alcohol for you if you are not allowed. But that's not absolute. If you go up to an adult in a parking lot and ask them to go inside the store and buy for you, you can both be arrested. But if you're planning an event where alcohol will be served, you can ask someone overage to take over the task of buying it (but you usually can't be with them when they buy it, unless it's your parent and they don't say it's for you).
In addition to breaking civil law, I'd say there would be religious prohibitions on getting someone else in trouble with authorities. Police routinely assign officers who are over 18 and under 21 to pose as ordinary citizens and ask random strangers to buy alcohol for them, then arrest them if they do it. Ditto for store staff who forget (or "forget") to ask for ID or who pretend not to recognize an obviously fake ID. So there is a strong risk of harming the person who tries to help you if you ask them to do this illegal act.
(Going back and forth about making this an answer vs a comment as I am not quoting halachot or religious opinions.)