You want to compare McDavid's season at 16 years old and 8 months to Lafreniere's 15 years old and 11 months? You think that would be a better comparaison? The difference is 9 months. The other way around we have a 3 month difference...
For all intents and purposes, Lafreniere was a 16 year old. Beyond that, development and growth doesn't occur in the linear fashion to make straight up age comparisons appropriate. In general, late-birth day players tend to be more physically developed for their age than pre-September 15th born players, and don't have as exponential an increase in stats as normal birthday players. Look at recent top picks with late-birthdays. Taylor Hall and Gabriel Landeskog are good examples, but far from the only ones. Taylor Hall, like Lafreniere, was a large for his age (though not quite as large as Lafreniere is currently) player, who played not just six games as a 15 year old but a month and a half. He only increased his point totals by six points in his second season. He definitely developed a lot as player in that time, but he was already very well physically developed.
Lafreniere is arguably more physically developed now than McDavid was in his draft season. You can chalk up a lot of McDavid's increase in points to catching up physically with the OHL. His skill and IQ always were always there, but what changed was physical maturity, going from a bantam sized player to a major junior sized player. Lafreniere is physically mature for his age, and he doesn't have to grow to match the league. That's why the comparison is bullshit. It tries to compare an older, far more physically developed player to a younger one who was undersized at the time.