In most multicellular organisms, every cell carries the same DNA, but this genetic information is used in varying ways by different types of cells. In other words, what a cell does within an organism dictates which of its genes are expressed.
Most enlisted positions require four years of active duty. If you join the Navy through an officer program, you must serve at least four of your eight years on active duty. However, pilots and flight officers must serve a minimum of eight to 10 years of active duty. Dentists, Medical Service Corps officers and nurses must serve a minimum of three years of active duty.
To serve as an Enlisted Sailor, you must be no older than 34 but at least 17 years old. If you are not yet 18, you must have parental consent. To serve as an Officer, you must be no older than 35 but at least 19 years old depending on the program desired. (Waivers may be granted for positions in high demand.)