Look at the expression on his face! "you lookin at me?"
Sorry to hear he gave you a scare. Our girl, Sophie, at home usually goes for 2 or 3 'walkabouts' per summer. She'll disappear for 4 - 5 days and pop back in looking fresh as a daisy. My guess is rabbit -stalking. It does give such an awful worry though, but so good when you hear the cat-flap go!
Sophie is also 12 and I would say that isn't too old. Although each cat is different, so if he's starting to slow down a bit it might be wise to restrict his outgoings as Angie suggested. My worry would be getting into scraps with other cats... Sophie's a bit of a bully, but one day she won't be as agile as she used to be!