Age Ain't Nothing But a Number received generally favorable reviews from critics. Many noted Aaliyah's vocal ability and praised the lyrical content. Aaliyah was credited for redefining R&B by blending her voice with Kelly's new jack swing. The album peaked at number 18 on the Billboard 200 and sold over three million copies in the United States. The album was certified Gold in Canada by the Canadian Recording Industry Association (CRIA) and Gold in the United Kingdom by the British Phonographic Industry (BPI) to date. The album has sold over six million copies worldwide.
After Barry Hankerson, Aaliyah's uncle, signed a distribution deal with Jive Records, he signed her to his Blackground Records label at the age of 12. Hankerson then introduced her to recording artist and producer R. Kelly. He became Aaliyah's mentor, as well as lead songwriter and producer of the album. Aaliyah and Kelly began recording the album in 1993 when she was 14. The duo recorded the first song, "Old School", at the Chicago Recording Company in Chicago, Illinois. While recording the album, Kelly coached her as they worked several hours in the studio. She often sang the songs multiple times in order to achieve "excellence". He claimed that Aaliyah was "one of the best young artists" he had worked with.
Aaliyah sang her songs in a falsetto. She felt that the songs "came at you kind of tough, a bit edgy, hip-hop, but the vocals can be very soothing". She went on to say, "lyrically I want things to be different."
A month before the release of the debut single, "Back & Forth", Aaliyah attended the Urban Network's "Power Jam" conference, where she was introduced and "received warmly". "Back & Forth" peaked at number five on the Billboard Hot 100 while topping the Hot R&B/Hip-Hop Songs chart for three weeks. The song was certified Gold by the RIAA. The second single, "At Your Best (You Are Love)", peaked at number six on the Billboard Hot 100 and was also certified Gold by the RIAA. In promotion for the album, Aaliyah embarked on a world tour that lasted from 1994 to 1995, she toured through the US, Europe, Japan and South Africa
Age Ain't Nothing But a Number received generally favorable reviews from music critics. Some writers noted that Aaliyah's "silky vocals" and "sultry voice" blended with Kelly's new jack swing helped define R&B in the 1990s. Her sound was also compared to that of female quartet En Vogue. Christopher John Farley of Time magazine described the album as a "beautifully restrained work", noting that Aaliyah's "girlish, breathy vocals rode calmly on R.Kelly's rough beats".Stephen Thomas Erlewine of Allmusic felt that the album had its "share of filler", but described the singles as "slyly seductive". He also claimed that the songs on the album were "frequently better" than that of Kelly's first studio album, 12 Play.
The album debuted at number 24 on the Billboard 200 chart, selling 74,000 copies in its first week. The album ultimately peaked at number 18 on the chart. It sold three million copies in the United States and 6 million copies worldwide. In Canada, the album debuted at number 29, peaked at number 20 and spent 25 weeks at the chart, selling over 50,000 copies and was certified gold by the CRIA. It also peaked at number 44 on the Dutch Albums Chart; it peaked at number 23 on the UK Albums Chart, where it was certified Gold by the BPI.