Combat soldiers are now younger than there were in many major U.S. and U.K. wars. I've edited away almost everything but the raw factual data in this op-ed piece from the Military Times about the age of soldiers in U.S. and U.K. military service quoted below.
As counter-intuitive as it would appear, the average age of young men dying in combat has decreased substantially from the mid-twenties of earlier wars to the late teens. One estimate puts the average age of Union soldiers during the American Civil War at just under twenty-six years old. A report on enlisted men from North-West England in the Great War puts the average age of dead soldiers at twenty-seven years old but more nineteen year olds died than any other age group. By World War II the average age of the US fighting man was still twenty-six years. In the Vietnam War the average age of soldiers killed in action had declined to 23 years and twenty percent of those killed were less than twenty years old. . . .
A snapshot of this evolving paradigm came on 10 July 2009 when five UK servicemen with an average age of twenty years were killed in the very dangerous Helmand province of Afghanistan. Three of the dead soldiers were only eighteen. . . .
SFC David D. Hack, US Army (Ret.) came up with this analysis of Vietnam War casualties:
• Five men killed in Vietnam were only 16 years old
• Of those killed, 61% were younger than 21 years old.
• 11,465 of those killed were younger than 20 years old.
• The average age of the 58,148 killed in Vietnam was 23.11 years. . . .
As these statistics show, notwithstanding lyrics of a popular song to the contrary, neither the average nor the median age of soldiers who died in Vietnam was nineteen (about 20% were nineteen or younger). The median age of U.S. soldiers dying in Vietnam was 20 and the average was 23. But, this was much younger than in prior conflicts.
The same source notes that both the U.S. and U.K. military allow both quite old and quite young people to enlist in the military.
The U.S, Military seems to agree with the concept of older recruits with a policy change by the Air Force in June 2014 when the maximum age for enlistment was raised from 27 to 39 years of age. The enlistment age restrictions for the Army, Navy and Marines are 35, 34 and 28. In a version of the moving goal post of necessity the U.S. Army announced in 2006 that they were increasing the maximum enlistment age to forty-two years old, the second time in six months having previously raised it to 40 years old.
The UK Ministry of Defence raised the enlistment age limit for its Army Reserve in November 2014 to 52 from 43, individuals with specific qualifications or experience 45 to 50. The announcement included references to older soldiers sharing knowledge and experience as well as widening the recruitment pool. It was stated that this was part of a plan to reduce the regular Army by bolstering the number of Reservists. . . .
The United States will allow enlistment of a seventeen year old teenager into their military with parental consent and eighteen year old without that consent. There are many aspects of law where certain individuals such as underage girls and more recently, boys, are incapable of consent. Surely this should apply to an equally dangerous scenario with even greater potential harm to the underage person.
There is a greater debate in Britain than in America over the minimum enlistment age of boys into the military perhaps because the age is so low in the United Kingdom. A boy can apply for military enlistment in England at the age of fifteen years and seven months with parental consent to join at sixteen years old. . . .
One realistic point of view is that a large majority of potential underage recruits fail early for health, academic or criminal history reasons. A senior military officer put the actual recruitment pool at less than five percent of the available under twenty-five year olds and even down from that for deduction of failures in the Armed Forces Qualification Test.
The term "available under twenty-five years olds" is a weasel word above, that probably omits men under the age of twenty-five who are in college or employed full time in a decent "real job" and hence aren't interested in joining the military.
While physical fitness is certainly relevant to military service, it also isn't clear to me just what share of positions in military service actually require the level of fitness and physical ability that is predominantly found in young men to perform their duties. Certainly, there are some, but a lot of military jobs don't obviously require this, particularly in the modern Navy, the modern Air Force, and in non-front line combat posts (e.g. diesel mechanics and computer system operators).