I used to have caffeine as a regular part of my life, but for several years now I've cut way back. I used to be able to hang out with the guys and we each drink our own 2-liter of Faygo, frequently Moon Mist (their knockoff of Mountain Dew). We'd stay up all night doing our thing (whatever thing it was that night). I didn't sweat it.
I don't attribute this to getting older, just to getting wiser. At some point I started realizing that I didn't always want to be under its influence, so I started indulging in the lower-caffeine alternatives instead of the colas or the fluorescent-yellow precipitationally-named sodas (though since I'm talking about my Michigan years I guess I should say "pop" ;) ). Now I don't drink caffeine unless it's because I need a specific boost, or if it's with the old crew in Detroit on occasion (I guess that makes me a "social caffeine drinker")--but the last time I did that to a significant degree (some year ago), I really felt it. Again, I don't call it getting older--I'm sure I can handle it as well as I ever did--it's just that now I'm not as acclimated to getting that frequent dose, so I noticed it way more, and I decided I didn't like it. I've heard that using caffeine continually means it doesn't give you the boost it would if you used it sparingly, so that's another reason to lay off of it.
Then over the years (especially since I moved out here) a similar thing has happened where sugar is concerned. So now I don't drink soda much at all any more. Mostly it's milk or water, with Powerade at home for an occasional changeup, or sometimes juice. But I'm not knocking down two- (or three!) liter bottles any longer--and certainly not at one sitting.
The biggest exception is Vernors, which I've started finding outlets for around the Metroplex. It's amazing: I almost never drank it when I was growing up there. I think I went 5-10 years with nary a sip. (Much of that probably was that no fast food restaurants that I can think of ever served it.) It's just something we took for granted. Then I moved away from Michigan for several years, and somewhere in there I took a liking to Canada Dry's ginger ale, but once I tried some just Vernors for old times' sake, I decided I missed it! I've spoken to other expat Midwesterners about this and every one has spoken fondly of it and said they missed it. Then I get to spring it on them that they can get it around DFW now. In one case I bought two 2-liters for a couple as a surprise Christmas gift. They were ecstatic... and my Mom who was visiting from Detroit watched the whole scene and said, "I don't believe this!" It was hysterical for me because I've been on both sides: missing it and unable to get it, and taking it for granted and forgetting about it.
Boy, that was a nice ramble... So I occasionally wind up with a 2-liter of Vernors in the fridge, and unlike most other sodas that might find their way in there, it usually doesn't go flat before it's gone nor sit there for a long time unopened. Other than that, soda seldom makes its way into my system.
I never could get into coffee, so soda is my only source of caffeine. (Penguin Mints just don't have that same appeal.) Mom was addicted to Starbucks for a while--she got to the point where she started getting shakes when she drank it, so she quit. It was hard on her but she didn't think she had much of a choice.