Is retposxto and retmesagxon the same?
Not really. Retpoŝto is the whole E-mail service. Retmesaĝo or retletero is the single digital letter you get via the E-mail system.
Do young people use email much these days? The sentence sounds kind of out of date to me, like grandfathers now use email, and youth use snapchat and whatsapp and such, or just text on their snazzy smartphones.