John was two days too old to be the youngest to play in the competition.
Glasgow Rugby's official website reports that John was 18 years 42 days old on Friday whereas Aled Brew was two days younger when he made his debut of the Ospreys against Cardiff Blues on 18 September this year. For that information, thanks are due to Stuart Farmer, statistical guru for the Celtic League and the Zurich Premiership among others.
A colleague of John's, Andy Hall, is among the front-runners for a different Celtic League record - the first to appear in 50 games in the competition.
Currently, he is in a five-way tie with the most career appearances in the Celtic League. He has played in 43 games, as have Gareth Wyatt (for Pontypridd, Celtic Warriors, and Dragons), Barry Williams and Andrew Millward (both for Neath and Ospreys), and Brian O'Meara (for Leinster). Some time early in the New Year one or more of them will reach the half-century.