You don't have a choice in the matter, the cold hard truth is that sooner or later we're all going to hit it... the big 40! Something magically happens with the infamous birthday - but it's not a massive biological or physical change - it's a psychological game really. Fitness over 40 isn't all that much different than "fitness under 40" to be honest. Listening to your body and understanding how it responds to exercise and lifestyle decisions is something you should be practicing at any age. A cohesive mind/body connection is a crucial component to have with any workout routine.
Don't use your age as an excuse for any reason! At 23 your body may still be a fat burning machine. You may think you can eat what you want, workout if you want, and sleep when you want and you'll still wake up feeling great and jumpin into those skinny jeans with ease. If you don't change your unhealthy lifestyle habits you'll be faced with a new reality when you hit 33, 43, 53 or any age. The fact of the matter is that the body is constantly changing and evolving. As time goes on, the body does indeed lose it's efficiency in repairing itself. It's an unfortunate fact of life. There is no rule book for fitness and age or fitness and ability for that matter. You shouldn't be comparing yourself to other people - you should be comparing yourself to YOU.