Fielding Old Men and Boys

"The Liberation Army does not want for willing fighters of its own. However, most are old men and inexperienced boys whose passion far outstrips their skill."

- Fire Emblem: Radiant Dawn

An army is so desperate for manpower it has to recruit (or draft) old men and Child Soldiers so as not to completely fall apart. This is generally the result of having sustained serious causalities.

Literature

 * This is the premise of the Discworld novel Monstrous Regiment, with the twist that . Early scenes in the novel include one where the last quartermaster at headquarters is closing up the warehouse because he's needed on the front lines more than the army needs a quartermaster corps.

Tabletop Games

 * Miniature wargame Bolt Action notes this occurred with the German "Last Levy" (forces protecting Berlin during the final Soviet offense) in World War II and has the option to deploy "Hitler Youth" units. What makes this a tabletop Games example instead of merely a historical one is that Warlord Games has an official miniature set for this "Theater Selection" containing six separate child soldier models, notable since child soldier miniatures are extremely rare across the medium's entire history, even when they exist in lore. Curiously, these children benefit from Germany's special rule "Initiative Training" like any other German unit, so half the time when their adult NCO is killed one of the kids can step up and do just as well as he could! Japan is also given rules for "Militia Squad" for its hypothetical anti-home island invasion force, but no miniatures have been produced as of this writing.

Theatre
"As I write these words, the enemy is plainly in sight beyond the river, and I begin to notice that many of us are lads under fifteen and old men, none of whom can truly be called soldiers."
 * Invoked in the last of General Washington's dispatches in 1776, written as he is encamped on the Brooklyn Heights watching the British approach:

Video Games

 * If you read the between chapter newpaper in Valkyria Chronicles it mentions Gallia steadily increasing the maximum age of conscription during the course of the war because of this. The only indication of children being fielded however is a 12 year old girl, explicitly the youngest fielded, who had to push to be accepted.
 * In Fire Emblem: Radiant Dawn thanks to Daein's military defeat three years prior and the remaining men being kept in prison camps, the Daein Liberation Army is said to consist primarily of old men and boys. This is consistent with the characters who join your group during their segments, as the only men are very young (Edward, Leonardo), relatively old (Nolan, Tauroneo), out of the country when the occupation started (Zihark, Sothe) or foreigners (Aran, Tormod, Muarim, Volug).

Real Life

 * Paraguay in the War of the Triple Alliance. This got so bad that it is believed 70-90% of the male population died during the war (the range is due to being calculated primarily from the post-war male-female ratio, but it is known that a significant number of women were killed during the war as well)
 * The Confederacy was forced to "rob from the cradle and the grave" just to keep forces fielded in the latter half of the conflict.
 * The line quoted in the 1776 example under Theatre, above, is a quote from an actual dispatch from George Washington to the Continental Congress. The two main surviving accounts of enlisted soldiers in the war come from Joseph Plumb Martin, who joined at fifteen years and seven months, and Ebenezer Fox, who joined a ship's crew at the age of twelve.
 * In 1945 the Wehrmacht raised Volksgrenadiers ("people's grenadiers", or basically militia when they were out of troops. Their real purpose was for them to die delaying the Allies to give the Nazi leadership time to get out of Dodge. They actually did their duty quite well in that department and they at least had a rational purpose of their own because they were also protecting civilian refugees from East Germany.