When L and Soichiro are surveying the Yagami household L says. "When I was young I did strange things too", which indicated that he is considerably older than Light was at the time (17). However he is able to pass off as a student in Light's final exam. He was old enough, clearly to have been training Near to be a detective for what could have been near's whole life (13) so just how old was L?
L's age was not mentioned in anime but in the prequel light novel Death Note Another Note, his age was mentioned as 22.
I don't have screenshot of night novel with me now, will try to come up with it if found any.
So his age in the main Manga/Anime is speculated to be 24-25. Same guess is present in deathnote.wikia.com.