Required enlistment age typically refers to how old an individual needs to be before he or she can qualify for military service. In some countries there is a compulsory enlistment age, at which point each citizen is required to perform some type of military duties or civil service. The age of enlistment differs from one country to another, though it is often closely related to the age of majority. In the United States, an individual must be 18 years of age to join the armed forces, though parental consent can allow for early enlistment at 17. Other countries have a required age anywhere from 16 to 22, and some require a high school diploma.
The two types of military enlistment are voluntary and compulsory, and they will sometimes have different required ages. Some countries have compulsory enlistment at the age of 18 or 19 but allow an individual to join earlier of their own accord. In these cases, anyone that takes early enlistment will typically avoid the compulsory process once they reach the appropriate age. Other countries with voluntary enlistment may also allow early enrollment. Canada has a voluntary enlistment age of 18, allows early enlistment at age 17, and enrollment at military colleges once an individual has turned 16.
Many countries offer similar programs to Canada's that can allow an individual to either enter the military early or get a head start on enlistment. The United States allows enlistment as young as 17, though there are reserve officer training corps (ROTC) programs for younger individuals. Junior ROTC is a program sponsored by the United States military that high school students can join. This is not actually military enlistment, though it provides some of the same structure and training. It can also serve as a stepping stone to the ROTC in college, a military academy, or enlistment at the age of 17 or 18.
Various circumstances can lead to a change in enlistment age regulations. Wartime or a shortage of soldiers can drive the required age lower, and some countries have regulations that specify this. Bolivia has a compulsory enlistment age of 18 with provisions to conscript individuals as you as 14 if the need arises. Greece also has a stipulation that the required age can be dropped to 17 during time of war, despite having a compulsory age of 19 at other times. Some countries enlist much younger children, though these actions are often neither official or legal.