I've read some about how Spartan training began when future soldiers were seven years old and was extremely tough -- a good number of recruits died before they even saw battle. Are there other examples in history of highly intense military training regiments?
Cannot confirm the physical toughness, but Sun Tzu's method of training was supposed to be mentally exhausting/tough.
Note: Sun Tzu was a Chinese military general/philosopher who supposedly lived around ~500 BC.
An example of this is in The Art of War, Tzu suggests in training to make sure that your men become attached to you. Once a general is confident of this attachment, he must break them and punish them, or else they will be 'useless'. Many of the techniques laid out in The Art of War include psychological strain to produce discipline. Many of these techniques are still used today by modern militaries.
My favorite legend about his training techniques is the following:
In one of the legends about Sun Tzu, he is tasked with training a large number of housemaids, as a joke from a powerful leader at the time. The leader wants them to be ready for battle as elite warriors - a task force to be feared by the enemy. The leader gives the task thinking it impossible. Sun Tzu spends some time working with the maids, and they are disobedient, making a fool of Tzu. They play practical jokes during training, despite physical punishment (the punishment can vary from version to version). So, naturally, he kills one of them brutally and very passionately in front of all of them, and orders them to perform their drills with the dead body still there. They execute the drill to perfection, and he declares the unit ready for battle.