If the song has an older version than that of Johnson and Butterfield, I am unaware of it. The following is an account of how it came to be written in 1864. The references after the title refer to recordings made by John McCormack. There are several songs which seem related, however.

When You and I Were Young, Maggie (George Washington Johnson/James Austin Butterfield)
B 23525-1, -2 (11 December 1919)
BVE 23525-3, -4 (17 December 1925)
broadcast (1 January 1926)
Notes: According to the notes by Philip Lieson Miller for RCA LP ARL1-1698 ("When You and I Were Young Maggie." Robert White, Tenor), this song commemorates one Maggie Clark, born in Glanford, Ontario. George Johnson also was born in this area, where he eventually became a teacher in a local school. The two became engaged and eventually married. The song alludes to features in the countryside there, including an old sawmill located on a creek near Maggie's home. After marriage the two moved to Cleveland, but Maggie died less than a year later (in May 1865). She was buried near her old home, and Washington too came home to Canada, where he was a Professor at the University of Toronto. The poem was first published in 1864. After his wife's death, Washington arranged for it to be set to music by Butterfield, who then lived in Detroit. He was a music teacher and minor composer, whose numerous other works are largely forgotten. The poem and the song attained great popularity in post-Civil War America. Maggie's sister published this background information in 1941, in response apparently to various erroneous tales of its origins that had circulated.

Incidentally, the site (by Paul W. Worth of Manchaca, Texas) which I got this from is an excellent resource on John McCormick, his life and recordings. I recommend it highly.