The song mmmbop was first released in 1996 as an independent album before the brother Isaac, Taylor and Zac were signed. Depending on when in the year the song was recorded the ages are as follows:
Isaac (Nov 17) 14 or 15
Taylor (Mar 14) 12 or 13
Zac (Oct 22) 9 or 10.
If your referring to the major label release they were Isaac 16, Taylor 14, and Zac 11.
If your asking they age when they last sang mmmbop, well as a fan favorite it is sung at almost every if not every concert. So as of 2013 the song is still being sung by the band as their new tour (and album) happens this summer.
Isaac would have been 16, Taylor would have just been 14 and Zac was 11
Jason Hanson is 39 years old
Isaac Hanson: 27 Taylor Hanson: 25 Zac Hanson: 22
As of the 2014 MLB season, Alen Hanson is 21 years old.
Travis Hanson is 30 years old (birthdate: January 24, 1981).
Brooke Hanson is 33 years old (birthdate: March 18, 1978).
Jason Hanson is 41 years old (birthdate: June 17, 1970).
Hart Hanson is 54 years old (birthdate: July 26, 1957).
Susan Hanson is 68 years old (birthdate: February 2, 1943).
Clarke Isaac Hanson is 37 years old (birthdate: November 17, 1980).
Jordan Taylor Hanson is 35 years old (birthdate: March 14, 1983).
Howard Hanson was born on October 28, 1896 and died on February 26, 1981. Howard Hanson would have been 84 years old at the time of death or 118 years old today.
Lars Hanson died on April 8, 1965 at the age of 78.