Check the eyes: closed or just opening usually means under 2 weeks; fully open but blue usually means 2 to 6 weeks
Check the ears: folded and small against the head usually means under 2 weeks; upright and alert usually means over 3 weeks
Check mobility: crawling only usually means under 2 weeks; wobbly walking usually means 2 to 3 weeks; steady walking usually means 4 weeks or older
Check teeth: no teeth usually means under 2 weeks; baby incisors appear around 2 to 3 weeks; more baby teeth appear by 4 to 6 weeks
Check size and weight: very small and under 1 pound usually means under 4 weeks; about 1 pound per month is a rough guide after that
Check behavior: constant sleeping and nursing usually means very young; playful and exploring usually means 4 weeks or older
Check coordination: poor balance and unsteady movements usually means younger than 4 weeks; better coordination usually means older than 4 weeks
Check weaning: starting to eat soft food usually means around 4 to 5 weeks
Check independence: using a litter box and eating solid food well usually means 6 to 8 weeks or older
Have a veterinarian estimate the age if you need a more accurate answer