There has always been a distinction between minors and adults in constitutional law. 1st amendment is best example. Free speech is different for children. And the court would treat 2nd amendment the same. By yiur reasoning, the congress could also say you have to be 21 to votes. Sorry, but I am confident you are wrong.Nope. The Constitution says very little about the age of majority. Originally it was only in reference to when you could hold a certain office. Later the voting age was lowered to 18 by an amendment.
The age where people could buy handguns and long guns was established by CGA '68, so Congress can change that at will.
And yes, other rights are age-limited as well, just different ages. School kids have a limited right to free speech, and same for search and seizure.
The only way the age cutoff of 21 would be allowed is to limit it to a certain type of gun, like AR15s or AKs, etc. But there is no way this Court would allow a complete ban on guns for 18-21. Ain’t happening.