The younger you are the more impact a healthy lifestyle will have on you. The best time to start is before you are even conceived. Parents should eat very healthy for at least 6 months prior to fertilization for the most impact on the correct development of an unborn child. The younger we are the stronger our hormones and DNA are, with the ability to construct super health and super humans.
People that start training and living healthy at their mid 20's will possibly have a very hard time undoing some of the damages they have already acquired. Their health potential has decreased by then. Of course it is never too late to get healthy and fulfill your health potential at the point where you are at.
But generally speaking there is a great point for early education of young people by directing their attention to building themselves up for a more healthy fulfilling life with the ability to enjoy living in their bodies. The sooner we wake up from the destructive habits we cultivated the better! It's especially sad to see an older person sticking to destructive habits like smoking or drinking, ignoring all the signs and symptoms of deteriorating health, making them seem so irresponsible it is a form of suicide. We sometimes see that with a friend or a relative, at a point where it has gone too far and it is beyond our ability to help or effect that person, it can be painful to observe.
Destructive habits however don't end with smoking and drinking. It goes all the way down to what and how we eat as well as our activity level (including how balanced that activity is). Eating unhealthy food is obviously very damaging to every part of our body from the way we look to our digestive system. With poor digestion comes poor ability to absorb nutrients which leads to shrinking of vital organs and even the brain!
Some people never wake up and realize the importance of healthy living. The consequences are a loss and a person who doesn't know themself, a person that hasn't reached their health potential, and a person that thinks the joy of destructive habits is bigger then the joy of health.
Our society as a whole is a sick society with the majority of people having severe health conditions at a very young age, you can see it by going out to the street. People try to get healthy by following the wrong advice that only causes them to get worse. The advice you will get about health from a sick society will generally not be very helpful. People try to hold onto bad eating patterns by finding a way to make excuses as to why it is healthy, like the concept of eating in Moderation (which I also wrote about in 2012).
You should research what being healthy truly means. The animals in the wild are generally healthy. We should find out what part of the planet we have developed in, what sort of diet was consumed at that part of the world for the past thousands of years, and what type of lifestyle have we developed to survive. I promise you it is very different then today's world, but many times bringing back the old and connecting to our true roots can bring back great ideas.
Health is a journey, we can always find ways to better ourselves, and doing better is fun and fulfilling. The other option is doing worse and there is nothing inbetween. The place between better and worse is a temporary plateau, and after that come worse. Maintaining is an illusion. Fulfill your health potential!