Two things related to this...
I heard someone talking about this after Tatum's injury, and they made the point that an Achilles injury is a wear-and-tear injury -- it gets weakened over time before it lets go, as opposed to just a sudden catastrophic failure. That's why it is traditionally older players who have suffered them. But AAU players are playing so many competitive games now, team doctors are seeing rookies come into the league with wear on their bodies they only used to see in vets.
Also, probably worth noting that the two young guys to blow an Achilles this postseason - Tatum and Halliburton - both spent their offseason with the Olympic team, and had been going for essentially two years without any kind of break.