Given this reference text, how old was Trey Anastasio when he co-founded Phish?

Ernest Joseph "Trey" Anastasio III[1] (born September 30, 1964[2][3]) is an American guitarist, singer, and songwriter, best known as the lead guitarist of the rock band Phish, which he co-founded in 1983. He is credited by name as composer of 152 Phish original songs, 141 of them as a solo credit, in addition to 41 credits attributed to the band as a whole.[4] In addition to his work with Phish, Anastasio has released 11 solo albums, and been part of several side projects including the Trey Anastasio Band, Oysterhead, Ghosts of the Forest, Phil Lesh and Friends, and Surrender to the Air.

Based on this passage, Trey was most likely 19 years old. He was born at the end of September in 1964 so there is a 25% chance that he was 20 years old when he co-founded Phish.