Follow up to this M.Y. question...
In most states, people under 18 aren't allowed to buy alcohol, because the law wants to discourage young kids from consuming it. The buying, per se, is not the problem.
Is there a halachic problem if you get an adult to buy the product for you? What halachot might you have violated and what halachot has the buyer violated by doing this act?
That would fall into Dina dimalchusa dina, but generally a law that is in place but it is common knowledge that it is not followed and everyone breaks it, is not assur.
As a matter of secular law your premise is incorrect. In the U.S. alone laws regarding minor alcohol consumption are extremely varied. In Wisconsin, a parent can legally buy their child a drink in a bar; in Pennsylvania it is illegal to serve your own child alcohol in your own home. There are many states in the middle.
In terms of halakhah, I would think dina d'malchuta dina would track whatever the local law is. If it's illegal to buy alcohol for the minor, then it's also asur and asking the adult to do it seems like lifnei iver (placing a stumbling block) and/or mesayeah (aiding and abetting) the Torah violation. If it's legal, then why would it be asur to ask them to do something legal for you?
The scope of dina d'malchuta seems beyond this question. But two points seem very relevant. (1) There are opinions that dina d'malchuta depends on enforcement. Shut Michat Asher 2:123. But the premise that underage drinking laws are not enforced seems dubious. I think there would be a distinction between laws that are not enforced and laws that are enforced but cannot be enforced in all cases or do not catch all violators. (2) While there are opinions that dina d'malchuta only applies to tax laws or other narrow categories of law, see Beit Yosef to Choshen Mishpat 369, most poskim hold that it applies to laws for the general betterment of society and prevention of mischief. See Rema 369:11; Igrot Moshe Choseh Mishpat 2:62; Aseh L'kha Rav 2:68.
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.)
