Autumn of Their Years features 26 songs recorded by the Beau Brummels from 1964 to 1966. The compilation contains unreleased songs and alternate takes from the band's Autumn Records period. Ten songs—including "She Sends Me", "Dream On", and "Love is Just a Game"—previously appeared on the 1982 Rhino collection From the Vaults, while the remainder of the album consists of sixteen unreleased tracks, including demo versions of the band's early singles "Laugh, Laugh", "Just a Little", and "Still in Love with You Baby".
Richie Unterberger of Allmusic wrote that the album has "a lot of fine moments, but actually a bit much for all but hardcore fans", and that "the best cuts ... were already available on From the Vaults". Regarding the previously unreleased demos, Unterberger said they "aren't as good, production-wise ... or material-wise", and called "Tomorrow Is Another Day" the highlight of the album, "showcasing Sal Valentino's rich and moving vocals".