Literally: old – young – man – woman
Alternately: All people. Absolutely everybody, regardless of age or sex.
Notes: While it is not entirely wrong to read this phrase with the more standard character pronunciations rou.jaku.dan.jo, the all-Ns version above is generally considered more correct.
Searching for this phrase gives you SO MANY stock images of generic collections of people.
I love the written word more than anything else I've had the chance to work with. I'm back in the States from Japan for grad school, but still studying Japanese with the hope of becoming a translator -- or writer, or even teacher -- as long as it's something language-related.