Yes. You must be 21 years old, not prohibited from buying an ordinary firearm. There is a process to go through to have the transfer of the suppressor approved by the Federal Govt, and payment of a one time $200 tax.
Do a web search on "firearms suppressors"
Not legally.
Legally, no. You must be 18 to purchase a rifle or shotgun, and 21 to purchase a handgun.
you just buy the silencer
Yes.
Depends on Texas law.
Federal law is 18
Be 21 yrs old, and not prohibited from buying an ordinary firearm. You will need a dealer in the state of Texas that can transfer the suppressor to you- they can guide you in the paperwork to do that. You will need approval of your local chief law enforcement officer, fingerprints and photos for the background check, $200 for the one time tax on the transfer of the suppressor, and about 5-6 months to wait.
No such weapon, so no. However, there there is a M16A4. It is possible, but, highly unlikely that an individual can buy one.
Yes the background check for guns is the same in every state
buy adding a silencer attachment
No silencer is going to actually "silence" your airsoft gun since the sound is coming from the gearbox, not the barrel. But a good place to get a mock silencer would be Airsoft GI and Evike.com