As someone who (foolishly) only drank decaf until my early 40s, I can tell you that the mental difference when you start drinking caffeine is just amazing.
It is wonderful stuff. And not only can you finally think in the mornings, but provided you don't get heart issues from it, it appears to reduce your risk of Alzheimer's substantially.
Everyone is different. I'm in my 40s and throughout my while life have found that caffeine only has two effects on me: 1) If I have any later than afternoon, I can't fall asleep when I want to, 2) If I have too much at once (like a single diet pop), I get the jitters.
Otherwise, literally no effect. I sometimes drink tea in the morning for breakfast (a London fog) and it doesn't affect my cognition or mood at all.
But I accidentally ran this test on myself recently and didn’t realise until I got home and found my full cup on the bench.
It was disturbing to know how much I need it.