Not taking away from the sentiment of congratulations, there is a slight revision of history there.
McMahon played QLD and Aussie schools in grade 12, winning a swag of awards along the way. A few weeks later in some park footy warm up 7s tournament, playing for a some random team, he was identified by the Aussie 7s set up. By the end of the year he was contracted to and playing for the Aussie 7s at 17 and days after finishing high school (as a legitimate U17 player, not a repeater).
Like all 7s players (and why I think we still struggle) he kept seeking a 15s contract, unlike may of the other he received interest in the first year he was out of colts in QLD age groups.
Very few colts age backrowers get full Super Rugby contracts, and 7s is always going to pay more and over rule a academy spot. He was never in a chance to be in the Reds program as since he was in school, he has been contracted to the 7s program nor would he have taken a spot of our contracted back rowers, many of whom are only a year or two older (though I'd certainly have him over a few of them).
So really as far as forwards go McMahon would be up with Pocock and Gill in terms of their immediate success and professionalism post school. All three hold a "youngest" player record, but I think McMahon required few Super rugby caps to reach Wallaby level.