Hi J.,
I have a 16 year old cat and my hubby is a veterinarian, so I thought I'd give you a little feedback on our situation and see if it helps.
My cat is actually my childhood cat that lives with my parents up in the mountains. I left home 12 years ago, but he's always remained with my parents because he is an outdoor mountain cat. Like your Tigger, "Dandruff" is an indoor and outdoor cat. He has a litter box in the event that he HAS to use it, but typically if he needs to go he lets us know and we'll let him outside. Because my parents house borders national forest, and he is an all white cat (could get skin cancer easier living at the high elevation)sofor most of his life he has spent the days inside sleeping and his evenings outside on the prowl.
Because of their location, we have always worried about him getting caught by a bear or mountain lion, but he always loved going outside so we've never had the heart to cut it off completely. I must say though, we always considered him our "wildlife warning" because there are some nights he would go out an hour or so and then demand to be let in, while other nights he refused to go out at all. We know he must have had a couple close encounters - when he was A LOT younger he came home a couple times with some deep gashes that needed stitches, but somehow he has kept himself safe enough to see 16.
I often look at him and think the stories he would probably be able to tell us if he could talk! Like you, it scares my family the thought of him going outside. My parents have also said that in the last couple years (maybe when he turned 13 or 14???) he hasn't longed for the outdoors as much. When he does go outside, it's usually at dusk for an hour or two and then he'd rather be inside getting some attention from his people! (no more all-nighters.) My husband, being a veterinarian, is amazed at his health and energy. Dandruff still pounces around the house and plays like a kitten. His overall weight is good, and is fur still has a shine to it. My mom says that for the first time recently she looked at him and saw his age a bit more in his fur (sounds like he is starting to show a little age) but overall he shows no sign of slowing down, or any health issues we should be concerned about.
I understand your concern from 2 sides - I may not live at home, but I (as well as my parents) worry about our old man all the time. I live in HR now, and have a 5 year old calico that lives with us - she is strictly an indoor kitty, and always has been, and I would be worried sick with the thought of her going outside! My advice with our experience from Dandruff - as much as we didn't want him to go outside before, he was such an explorer we could never really hold him back. (where as my little city cat is such a princess she has no desire to go outside.) Like Dandruff, I think for Tigger to make it this long he's got to be smart and have an idea of what he's looking out for. If you are concerned, I would try to limit trips outside to the dusk/early evening period, and for a couple hours so you can watch him a bit more. Hopefully that will be enough to get the energy out for him. If he's anything like Dandruff, in the next couple years he will WANT to come in versus pulling all nighters - not because he's in poor health, but because he wants to be with his people.
We have not done anything "special" for Dandruff - with the exception that he doesn't spend the nights outside. He still eats the same food, and being married to a vet we've learned we watch his water intake - basically, we make sure that he is drinking the same "normal" amount. My husband will say to also monitor how often they are peeing, but of course that is harder when they go outside! Although he has had to have a few teeth pulled, at 16 Dandruff appears to have great listening/seeing senses still. I would say the only difference in is personality is he craves attention from his people much more!
Hope some of this insight on another "older" cat helps. As healthy as he is, every day I half expect a phone call from my parents saying he is gone due to age. I wonder if when his time comes he will go in mountains and not return home - not because he is caught by another animal, but because he knows it's time and finds a peaceful spot. But I have to tell you - even at 16, he doesn't show ANY signs of that day coming any time soon!
Good luck with Tigger!
M.