Iuvenis means "young man", yes, and, according to my dictionary, Cicero uses it to mean citizens who fit for military service. But Suetonius apparently uses it for "young people," regardless of gender; and Livy uses it to mean "young woman." So it seems to me that other acceptable translations here are:
(The) young people save (the) cities. (The) young women save (the) cities.
The verb servare means to save. It gives us such words as conservation, preservation, to preserve, to conserve. You're thinking of servire, which means to serve. It's a completely different word. :)
Double check on William Whitaker's Words to make sure: http://archives.nd.edu/cgi-bin/wordz.pl?english=serve
Volgav vitsenanieff nivya kevach varatsach.
I am really sick of iuvinels. It was ok at first when we were told it could also mean young people or young women. But thats not how the game is scored. Give us female young people who go to universitas and save cities and get used to it. Lingua Latina hasnt got an OED to florish at the proponents of masculating pronouns - and neither really does the OED. More complex linguistic sexuality is out there and it will find you. it will have fun. will you?