Because other systems lead to abuse where people are deemed "not X" for a variety of reasons including race, religion, gender, sexuality, politics, thoughts, and many other reasons.
All systems lead to abuse, including the existing system that artificially selects one age for a variety of rights/obligations with the notable exception being alcohol consumption. The rational approach would be to find a system that creates the minimal level of abuse. If that's the current system then so be it, but an effort should be made.
The original article refers to one instance with potentially negative consequences where the age of majority (18) is the basis for access. Why not, in line with the authors' suggestion, increase the age to 25?
