...and he ends up looking like Jimmy Kimmel.
from the commentary they say they even used prosthetics using molds from Willis's face
Using molds from Willis’ older face to make Levitt look like a younger Willis was always going to produce... questionable results.
The two didn’t resemble each other that much (understatement), albeit I kinda dig Levitt’s look to be honest. In a way, I feel it worked for this film.
How Looper Made Joseph Gordon-Levitt Look (& Sound) Like Bruce Willis
https://screenrant.com/looper-joseph-gordon-levitt-bruce-willis-face-effects/
Looper sees Joseph Gordon-Levitt and Bruce Willis play younger and older versions of the same character, which involved some impressive visual tricks.