Sounds like you've got everything pretty well under control. I don't know anything about the iron supplements, but if the vet suggested it, I don't think it would harm her any.
My Tubby is 16 now. He sounds like Ingrid in that there are really no outward signs of his age. A few years back I noticed that he was having a little harder time jumping to high places - like onto the bed. I had my dad make a box for him to jump on first, kind of like a step up to the bed. He doesn't jump on the counter anymore - which is a good thing.
Also a few years ago it seemed that he was throwing up a lot. He had always had hair balls, but it seemed like it was almost everyday he would throw up. Took him to the vet and he is in the early stages of kidney failure. The vet prescribed a 1/4 of a pepcid a day and that has been working wonderfully for him. He only throws up occassionally now, and that's usually because I give in and give him goodies he shouldn't be getting.
But other than these two things, he's as normal and active as ever. But I can definitely relate to your feelings about what is to come. I dread the day I have to make any tough decisions, but if he keeps up the way he's going, that day won't come for quite awhile, and I'll make sure to enjoy everyday I have with him until then. 
Here's a picture of Tubby checking out his kitty kube when he first got it. Now it's in the bedroom and he uses it everytime he jumps up on the bed. It only took two "lessons" and he had the hang of it.
Tubby
Spring 1986 - Dec. 11, 2004
RIP Big Boy
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Peanut
Fall 1988 - Jan. 24, 2007
RIP Snotty Girl
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Robin
Fall 1997 - Oct. 6, 2012
RIP Sweet Monkeyhead Girl
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