Fielding Old Men and Boys

An army is so desperate for manpower it has to field old men and Child Soldiers to not 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.

Theater
"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.