Kittens have small, razor-sharp teeth. Adult teeth start coming in around 6-7 months old.

Shiny, thick coats in kittens fade to duller adult coats. Senior cats may have thinning coats.

Kittens and younger cats are energetic and playful. Senior cats are lower energy.

Kittens and young cats have bright, wide-open eyes. Senior cats may get cloudy eyes.

Your vet can estimate age based on dental wear, eye appearance, muscle tone, and more.  

Shelters and rescues may have background on the cat's estimated age.

Generally, the first 2 years equal 24 human years followed by 4 cat years per human year.