You sneak into the hokages' office. Take a naked picture of them, that or them taking a shit. Then tell them to make you an anbu otherwise you'd pin the photo in every corner of Konoha. Just ask Itachi.
The shinobi in the ANBU are hand-picked by the Kage; chosen for their individual capabilities and special skills. Age, gender, background, or previous rank bear no weight in this decision.