According to the US Consensus middle-age ranges anywhere from 35 to 54. During this stage of life one begins to see visible signs of aging, loss of skin elasticity, and graying hair. In addition, physical fitness decreases, body fat accumulates, and a decrease in both aerobic performance and maximal heart rate ensues.
You may be asking yourself why any sane middle-aged person would ever want to enlist into the army, especially at such a physically disadvantageous age. Well...there ARE people out there who have their various reasons. One middle-ager, Russell Dilling, decided to follow through with his life-long dream and joined at the ripe age of 42 after divorcing a wife who refused to be in a military marriage. Another middle-aged aspirant of the military life posted, "I really wanted to be a cop or firefighter, but their qualifying standards are too tough. Also, cops and firefighters don't get the great benefits that military people do, like free health care, housing, discounts at the PX, job placement assistance, and various other preferential treatment", to which an incredulous military solider responded, "[I] think you have the Army mixed up with the Welfare system...". Hilarious.
In any case, it's no secret that soldiers experience long bouts of sleep deprivation during combat and perhaps even in training. This is why the U.S. military developed its very own sleep-reduction program. It seems as if the development of a drug to help cut down that pesky need for sleep is on the horizon (or already here). In an unclassified report the defense science advisory group known as JASON wrote:
So why might it be a bad idea for middle-aged adults to enlist into the army?
[T]he maximum casualty rate depends strongly on the individual's sleep need, τ0. Hence any effort to improve human performance to minimize τ0 for given tasks can lead to a significant decrease in the casualty rate, of [about] 20 percent. … Suppose a human could be engineered who slept for the same amount of time as a giraffe (1.9 hours per night). This would lead to an approximately twofold decrease in the casualty rate. An adversary would need an approximately 40 percent increase in the troop level to compensate for this advantage.
A recent study by Wigren, Rytkönen, and Porkka-Heiskanen over at the University of Helsinki investigated how aging affected the capacity of the basal forebrain (BF), a crucial area for sustaining cortical arousal, to cope with prolonged waking activity. They found that lactate, a chemical induced by neural activity, increased in sleep deprived young but not middle-aged or old rats. Additionally, they observed an attenuation of increased high-frequency (HF) EEG theta power (7–9 Hz), a marker of cortical arousal and active waking, in middle-aged and old rats.
The results suggest that age-related weakening of BF functioning reduces cortical arousal during prolonged waking. In other words, middle-aged and old rat brains just can't hang with that of their younger counterparts when it comes to sustaining wakefulness after a period of sleep deprivation. How is private Dilling going to stay awake during long hours of combat? What's even worse, sleep deprivation may reduce subsequent sleep intensity within this age group and further impair cognitive functioning (also check out how Sleep deprivation might lead to Alzheimer's).
Obviously further research on human subjects is needed before any definitive conclusions can be drawn. For now, those of you who are currently experiencing a mid-life crisis and want to prove that you've still got what it takes, I suggest avoiding enlistment and maybe purchasing a Harley motorcycle instead. Just make sure you get enough sleep before taking it out for a long cruise.
Wigren HK, Rytkönen KM, & Porkka-Heiskanen T (2009). Basal forebrain lactate release and promotion of cortical arousal during prolonged waking is attenuated in aging. The Journal of neuroscience : the official journal of the Society for Neuroscience, 29 (37), 11698-707 PMID: 19759316