No, there is no federal civil rights law that applies, employment yes but not public accommodations.
A business cannot refuse service to anyone that the Civil Rights Act deems a "protected class." This would include discrimination based on race, color, religion, sex, creed, national origin or disability but NOT age under the present Equality Act of 2010.
This is why such a law banning firearms to those under 21 would make such a great test case.
If you were around during WW2 President Franklin D. Roosevelt lowered the minimum age for the military draft age to 18, at a time when the minimum voting age (as determined by the individual states) had historically been 21.The debate about lowering voting age from 21 to 18 began during World War II and intensified during the Vietnam War, when most of those subjected to the draft were too young to vote, and the image of young men being forced to risk their lives in the military without the privileges of voting successfully pressured legislators to lower the voting age nationally and in many states.
In 1984, Congress passed the National Minimum Drinking Age Act, which required states to raise their ages for purchase and public possession to 21 where it remains today.
BUT buying a beer is not covered by the Constitution or the Bill of Rights...
But voting is - 26th Amendment
and owning a firearm is - 2nd Amendment.
So would passing a law or Presidential Order against a legally aged adult who can legally vote, legally marry, or can be legally drafted be denied their constitutional right to keep and bear Arms?