In X-2: X-Men United, Colonel William Stryker encounters Wolverine at the mansion.
Wolverine? I must admit this is certainly the last place I'd expect to find you. How long has it been? 15 years? You haven't changed one bit. Me, on the other hand... Nature.
As he says "nature", he taps the side of his forehead.
Is Stryker simply referring to the fact that he has aged naturally (with an oddly timed hand gesture), or does the tap of his forehead as he says this refer to something else?