More like in the original myth there were numerous Valkyries, and the named ones clocked at around 29 from the Eddic ones alone (the Wagnerian ones, including Rossweisse, was 12) Their names, at least the Eddic ones, were basically epithets, which in DxD, translates as their capabilities (for example, Gondul means "wand-wielder" in Norse, which makes it very fitting considering her magical abilities)
They're masters of rune magic after all, so it's entirely possible that they use it to keep their bodies fit unless they want to retire.
As for Gondul's appearance, Volume 17 described her as basically having Rose's body with the face of a woman in her fifties to sixties.
Rose's 18, yes (19 by the current volume).