Ages Ago is a musical entertainment with a libretto by W. S. Gilbert and music by Frederic Clay that premiered on 22 November 1869 at the Royal Gallery of Illustration. It marked the beginning of a seven-year collaboration between Gilbert and Clay. The piece was revived many times, including at St. George's Hall, London in 1870 and 1874, and in New York in 1880.

By the 1850s, the London stage had fallen into disrepute. Shakespeare's plays were staged, but most of the entertainments consisted of poorly translated French operettas, risque Victorian burlesques and vulgar broad farces. To bring family-friendly entertainment back to the theatre, Thomas German Reed and his wife Priscilla opened their Gallery of Illustration in 1855 and brought in Gilbert in 1869 as one of their many playwrights. The Gallery of Illustration was a 500-seat theatre with a small stage that only allowed for four or five characters with accompaniment by a piano, harmonium and sometimes a harp.

After Gilbert's first offering for the Gallery of Illustration – No Cards, with music by Reed, Gilbert paired with Clay on Ages Ago, the first of a successful series of collaborations between the author and composer that would continue for the next seven years. In the eight months between the productions of No Cards, and Ages Ago, Gilbert's dramatic style had developed. Ages Ago, with its double-layered plot and its complex relationships among the characters, is more sophisticated than No Cards, which was a simple farce. In addition, the lyrics move the plot forward more than in the earlier work.

Ages Ago earned praise from the critics, outran its companion piece, the popular Cox and Box, and was frequently revived over the next decade. It was Gilbert's and the Gallery's greatest success to that date, running for 350 performances in 1869. It was revived several times thereafter and is still performed occasionally. At the 1874 revival, Mrs. German Reed, Leonora Braham, Alfred Reed, Stanley Betjeman, Corney Grain, and the piece itself all received warm praise from the Era's critic. New York's reopened Broadway Opera House was inaugurated in 1880 with a double bill of Ages Ago and Charity Begins at Home.

Gilbert produced four more pieces for Reed, including A Sensation Novel in 1871 and Eyes and No Eyes in 1875. He also wrote several comic operas with Clay, the last of which was Princess Toto in 1876. Thomas German Reed played Ebenezer Tare, while his wife played Mrs. MacMotherly. The piece also introduced Fanny Holland, who would play in many pieces for the German Reeds for years to come.

At a rehearsal for Ages Ago, Clay formally introduced the composer Arthur Sullivan to Gilbert. The two later collaborated on fourteen comic operas that became the most enduring pieces of musical theatre from the Victorian era. Gilbert would later reuse many ideas and plot elements from these earlier works in the Gilbert and Sullivan operas.