If it wasnt one of the Essendon players in '93, it probably would have been someone in VFL days who was younger - random guess maybe Tim Watson in late 70's, or perhaps Dermie in 83 (84?) - so if we're talking VFL days, thats not even relevant to us, be better off mentioning Wangas in '90 SANFL Grand Final.
In saying that, I reckon Ashley McIntosh was fairly young when he played his first final - without looking it up, I'll say it was in '91. From memory, I think he was born in '73, so he was perhaps around Colquhoun's current age.
Thanks! Looking up the ages of the players you listed it looks like Fletcher was the last player younger than Colquhoun to play in a final. He missed the 1993 Qualifying Final with an ankle injury but played in the Semi Final on the 12th of September. Fletcher didn't turn 19 until May 7 the following year.