Thanks a lot, velisarius."An adult" is singular, so you can't use it for more than one person. "You" may be singular or plural, of course.
"They are adult" - who or what is "they"? Why are you using the adjective?
Thanks a lot, velisarius.I'm not saying your #1 is impossible, but I'd need a particular context for "They are adult".
If we simply want to say that certain people are over 18 years of age, we normally say "They are adults".
All my students are adults. They are adult learners.
Note: Please don't number your sentences as you did in # 1 and #3. It's confusing. Each different sentence needs its own number.