We aren’t sending them to war to die, there hasn’t been a draft in half a century
You can argue morally but legally there’s nothing in the constitution defining what an adult is except for the 26th amendment and that specifically talks about the right to vote. In the case of voting Oregon v Mitchell decided that it was unconstitutional to force states to lower there voting age to 18 for state and local elections without an amendment, which eventually was added. Barring another amendment passing why can’t states choose to decide what they define as adults for gun ownership?