You can make a pretty good estimate on a cats age by examining their teeth. When I worked at the SPCA, we used the cats teeth to determine an approximate age.

2-4 weeks

Deciduos incisors coming in.

3-4 weeks

Deciduos canines coming in.

4-6 weeks

Deciduos premolars coming in on lower jaw.

8 weeks

All deciduos teeth are in.

3 ½-4 months

Permanent incisors coming in.

4-5 months

Permanent canines, premolars and molars coming in.

5-7 months

All permanent teeth by 6 months.

1 year

Teeth white and clean.

1-2 years

Teeth may appear dull with some tartar build-up
(yellowing) on back teeth.

3-5 years

Teeth show more tartar build-up (on all teeth) and some tooth wear.

5-10 years

Teeth show increased wear and disease; pigment visible on gums.

10-15 years

Teeth are worn and show heavy tartar build-up; some teeth may be missing.

Information from: Rural Area Vet