Minecraft: Fall Damage

From the wiki

Players receive damage when falling from excessive heights. Armour does not reduce fall damage, unless it is enchanted with Feather Falling or Protection. Fall damage is 1 (half heart) for each block fallen after the third. Thus falling 4 blocks causes 1 half heart damage, 2 (full heart) damage for 5 blocks, 3 (heart and a half) for 6 blocks and so forth. Assuming full health (but no Feather Falling or relevant status effects), a 23 block fall will be fatal (23 - 3 = 20 (Full Heart × 10) of damage).

Mobs (other than chickens, ghasts, snow golems, blazes, magma cubes, iron golems, ocelots, bats and, presumably, the Ender dragon and wither) suffer fall damage as well.

A skeleton has 10 hearts, as well as zombies and creepers, so the fatal fall distance is 23 blocks for them.