system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Second Amendment: what is well - regulated?

As a European, it is hard to grasp the right to bear arms. Without blaming the US for having the Second Amendment, I just want to understand it. President Obama says he wants to do something about gun violence, but his chances are not that high (I guess). 

The Second Amendment says 'a well-regulated millitia'. What does that mean? Is it possible to impose restrictions based on this clause? For instance, prohibiting selling arms to people under the age of 21, or to people having mental illnesses or to convicted criminals?

Again, I don't want to question your constitutional right, I just want to understand it.
gpt
The second ammendment is a two parter and is seperated by the comma in it. The part concerning firearms and such is "... and the right to bear arms shall not be infringed." The "well-regulated" part concerns only the militias.