"Legal" denotes a law- and the law varies depending on where you are- and you did not tell us that. In most of the US, firearm suppressors ARE legal, but must be registered with the Federal BATFE, and a $200 transfer tax paid. Please note that for most rifles, a suppressor does NOT "silence" the rifle, but can greatly reduce the sound.
Yes
You need to tell us which rifle. Chances are that there is some legal way to own it.
18
Depends on where you are.
supress
The minimum legal barrel length for a rifle without requiring it to be tax stamped and registered as a Short Barreled Rifle is 16 inches - that's federal law, and not just Vermont.
subdue
You must be 18 to buy a rifle. The stock has nothing to do with it.
That is question for your state laws. Check local legal representation.
yes
yes
Yes