About T-Shirt Design
I'm not Old I am Classic T-Shirt works because it sounds more like something a real person would say than a random slogan. It fits really well with everyday humor, but you do not have to be obsessed with that stuff to enjoy the joke.
You could wear it for holidays, bring it out for birthdays, or just keep it in the normal shirt rotation. It works well as a novelty t shirt because the joke is clear and the design has plenty of personality. The whole point is to get a laugh, and the shirt does that without making things complicated.