"What's My Age Again?" is a song by American rock band Blink-182. It was released in April 1999 as the lead single from the group's third studio album, Enema of the State (1999). The song, written primarily by bassist and vocalist Mark Hoppus, lyrically revolves around the onset of age and maturity, and the failure to implement changes in behavior. A mid-tempo pop punk song, it was written by Hoppus based on personal experience. It was originally titled "Peter Pan Complex", an allusion to the pop-psychology concept, but MCA Records forced the band to retitle the song, as the label found it too obscure a reference. It was the first Blink-182 single to feature drummer Travis Barker.

It became Blink-182’s second hit single, peaking at number two on Billboard's Modern Rock Tracks chart for ten weeks. The song hit number three in Italy and number 17 in the United Kingdom. Primarily an airplay hit, the song was the band's first to cross over to pop radio, hitting number 58 on the Billboard Hot 100. The song's signature music video famously features the band running nude on the streets of Los Angeles. It received heavy rotation on MTV and other music video channels. As a result of this exposure, it gave the band an unwanted reputation for nudity, which would lead them to take more creative control of the group's image later in its career.

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