He's buddies with Charlie Sheen. Sheen talked about using hormones on multiple occasions in the past. Penn's been seen coming out of HGH clinics on multiple occasions...All those older Hollywood dudes are on HRT.
It's not just older actors, juicing for roles is quite common in Hollywood, HGH is huge as well as test/mast/winny. The "trainers" are pretty much the same type of guys as bodybuilding "gurus" but charge way more.
Hugh Jackman, Dwayne Johnson, Mark Whalberg, Christian Bale etc etc etc...