It's pretty straight forward estimating age in a dog or cat over 8-12 weeks and under a year as there is a progression and age of tooth eruption that is consistent (with a little variation in toy breeds - they tend to be slower).
Then it's easy to identify a geriatric mouth, but again that varies by breed, dental care during their life, life style and diet.
The hardest age to estimate is 1-7 (or to late adult - later in some breeds).
Sometimes the best you can do is say young, middle age, old and really old.
Doug, DVM