Translation:Are you really buying a sixteen-year-old car?
Seems like "šestnáct let staré auto" is a metaphrase from English. IMHO, the right variant is "šestnáctileté auto"
No, that is not correct. We sould use "šestnáctileté" mainly for living entities.
In my impression no Duolingo course accepts digits in the language studied (that would amount to cheating) but some courses accept them in the learner's language. The Czech course for English speakers does not seem to. I guess it's a philosophy.