On maps featuring the Cherokee or Klamath, it is possible for civilizations lacking Skirmisher-type units to gain them in the Colonial Age. The Cherokee Rifleman and Klamath Rifleman are comparable to Fortress Age skirmishers but with much higher hitpoints. Cherokee Riflemen get a +20% speed upgrade (applies to native warriors), Klamath Riflemen have +25% damage (applies only to their rifles).
Chinese and Indians can get Skirmishers by allying with British and Germans at the Consulate, which provides armies and shipments that includes Skirmishers (named Roger's Ranger for the British, Prussian Needle Gun for the Germans) listed below. These Skirmishers start with +10% hit points and attack than their regular counterpart, and are automatically upgraded in every Age starting from the Fortress Age (see here for the exact values).
"Skirmishers were lightly armed and armored riflemen who used their speed to move quickly about the battlefield and perform a number of military roles. They defended flanks, helped artillerymen with cannon, dragged wounded soldiers off of the battlefield, and performed hit-and-run attacks on advancing enemy armies. Napoleonic-era skirmishers were effective against rigid ranks of musketeers because the fire of musketeers was less effective on the loose, informal skirmisher lines.
Many skirmishers in the nineteenth century were armed with rifles; the range and accuracy of these weapons made skirmishers dangerous enemies. Early rifles could only be fired once every minute or two, though later breechloading and repeating models could fire much more rapidly. One model fired so fast, a commander ordered his men to focus their fire on a single enemy skirmisher so that they could recover his rifle to examine it."