My dog is two and he does the same thing. He enjoys being outside but hates to be shut outside-my french window is almost permanently daubed with muddy footprints where he's scratched for me to open the door. He actually wants the best of both worlds; to romp around the garden whenever he feels like it and to keep a check on what I am doing at any given moment. When I do open the door and invite him back in he stands about two feet away and refuses to budge or bounds off with a garden spade or some other forbidden item in his mouth (believe me he's that big and strong enough) so that I have to chase him around the garden(his favourite game!) I guess that, at twelve, your dog is very close to you and just wants to enjoy the outdoors but also to be back at your side whenever he wants. I know it can be really aggravating at times, and very impractical. None of which is helpful in solving your problem I'm afraid.