To the naysayers that can't think of any anime featuring main characters over the age of 16, I present this nice long list of anime that will have you think otherwise. :P
Jin-Roh: The Wolf Brigade Millennium Actress Perfect Blue The Girl Who Leapt Through Time (main trio of characters are in their final year of high school hence they are 17-18 years old) Read or Die Welcome to the NHK Black Lagoon Berserk Mushi-shi Gungrave Paradise Kiss Key the Metal Idol RahXephon Nana Honey and Clover Chobits Love Hina Nodame Cantabile Trigun Outlaw Star X/ 1999 Blue Gender Cowboy Bebop (if you don't lump in Ed) The Big O Video Girl Ai Ghost in the Shell: Stand Alone Complex Vampire Hunter D (both movies) Saikano Princess Mononoke Elfen Lied (Lucy, Kouta and Yuka) Armitage: Dual Matrix Full Metal Panic trilogy (Tessa, Kaname and Sousuke are all 16-17 years old) Hellsing Looking Up at the Half-Moon Porco Rosso Wings of Honneamise Excel Saga Hataraki Man Texhnolyze Paprika Otaku no Video Paranoia Agent Death Note Afro Samurai (for the present timeline) Monster Eden of the East Emma: A Victorian Romance Rose of Versailles The Tale of Genji Noir Suzuka The Sky Crawlers Spiral Samurai Champloo Baccano Genshiken Starship Operators Wolf's Rain Darker than Black (the first season and OAV) Durarara Arakawa Under the Bridge House of Five Leaves Planetes Cromartie High School Summer Wars Eve no Jikan Rin: Daughter of Mnemosyne Aoi Bungaku Series Pet Shop of Horrors
@rein: I normally keep tabs on all the titles I've completed by storing it online and written down in notebooks (or via my anime collection). Comes in handy when I'm trying to remember info on what I've watched.