A few decades ago implies 1990s or early 2000s. In 1990 he was 7 years old. In early 2000 - 17 years old.

a few

a small number of units or individuals

A few is at least 3 because then you'd use "a couple". So its 3 decades or more, which is 30 years at least.

This is not how natural languages work.

This is exactly how it works, and thread is proof of that. You're just being wrong and trying to find your way out of it.