CHARLIZE THERON GOOD GENES OR GOOD DOCS?

8/8/2015 7:46 AM PDT

Charlize Theron looks like a "Young Adult."

Here's a 21-year-old version of the "Mad Max: Fury Road" star at the Los Angeles premiere of "2 Days in the Valley" back in 1996 (left) and nearly 19 years later ... the South African actress -- who just celebrated her 40th birthday -- in L.A. last month (right).

She sure ain't no "Monster."

The question is ...