Rather than outright wrinkles, perhaps you could show signs of powerful expression and slightly weathered skin, such as crow's feet when he smiles or grimaces, and bags under his eyes. He has, after all, been through much more than pretty much anybody out there. I dunno if I compare him to Crono, though; Crono has been hit upside the head with a bazillion different projectiles, sucked dry by freakish mutants, zapped with lasers and magic, whacked in the face by lizard men, munched on by a giant dinosaur, disintegrated into nothing and then resurrected, had to fight Queen Zeal after she turned into something like Bekkler on crack, steroids, meth and a little acid, and then killed an immensely powerful alien tick by cutting through its body and killing its insides. And apparently, he seems to have died again, and has nothing to bring him back this time.
No wonder he's addicted to Ether.
But, I digress. If you don't want to make him look outright wrinkly, just put in features that show signs of stress and such.
