[Retroman]

I think this is just a simple case of time passing by quicker than people realised...

Combined with the fact Friends was constantly repeateded on our televisions for years on end, which obviously showed Joey/Matt always young. Plus we don't get as much chance to see him as perhaps Americans do.

He's going grey at a decent age, and not covering it up with lots of hair dye.
People go grey as early as twenty years old or so, so fourty isn't so shocking.

But yea, I think this is more of a "Wow, has it been so long?" situation.
Not a "Wow, has Matt aged really quickly?"

He does look a lot different though, complete contrast to what he used to.

He's gone from boyish good looks, to a rugged handsome look >_< or perhaps just an old and tired one.