What I've learned is that caffeine metabolism goes down with age and sleep gets lighter with age. Even if you can fall asleep easily, the residual caffeine in the middle of the night is enough to wake you out of light sleep. I made a tool to convince myself to cut back: http://jitterdone.com
What you could get away with in your twenties doesn't go unpunished once you're around thirty. And what you could get away with around thirty doesn't go unpunished once you're around forty.