Young people generally don't pay enough attention to their health and fitness. Do you agree?

Nowadays health and fitness are a matter of great importance and there is often a lot of discussion in the media about this subject, especially when referred in relation to young people.
It is usually told said that young people don't worry enough about their health and fitness; maybe they think of postponing this topic when put it off until they are older.

Teenagers often spend too much time in front of a computer, playing video games or watching television instead of doing regular physical exercises or outdoor activities. Furthermore, they usually have eat a small breakfast in the morning and often eat junk food during the rest of the day.

This kind of lifestyle can be dangerous over the years and lead to obesity and or heart problems, while working out on a daily basis and having a balanced diet can be beneficial to your health.

Nothing wrong with the way you have used "while" to introduce a contrast but I prefer "whereas". Many British writers would use "whilst" instead of "while".

Young people should remember that health is not a necessity you acquire whenever you want and once for ever, but something you achieve time after time and the earlier you start the better it is.

Young people should remember that good health is not a commodity that you can get whenever you want. Rather it is a luxury that you may be lucky enough to obtain and preserve by living well every day. The sooner you start, the better your chances will be.

Not sure if I captured what you meant in that last paragraph.

Great work Stefano!