Do a web search on "firearms suppressors"
Legally, no. You must be 18 to purchase a rifle or shotgun, and 21 to purchase a handgun.
Not legally.
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.
buy adding a silencer attachment
Yes the background check for guns is the same in every state
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