Kittens mature rapidly in their first three or four months. You can tell their age very approximately by several methods.
Difficulty: Moderately Easy
Step 1 - If they still have their umbilical cords they are probably between 1 and 3 days old.
Step 3 - Open their mouths and look for teeth. Do you see little nubs coming in? About 2 weeks.
Step 5 - Are they starting to play? Figure their age at about 4 weeks.