It is quite easy to tell, if a human is old, since it starts looking like a human-skeleton hybrid. Why is it so much harder to tell if an animal is old?

Because we are human?

Most mammals other than humans are covered with thick hair that hide skin wrinkles.

Most animals in the wild don’t live long enough to GET that old.

Domestic dogs, cats, horses on the way to the glue factory—can be very bony when they exceed their natural life span, or grow sickly.

I’ll bet the animals can tell. Penguins and seagulls can find their mates when they bring home food. How do they know?

Human beings have trouble making accurate assessments of age across race. You can’t really expect them to do any better across species.

Like the @LuckyGuy says, the animals that need to know are probably damned good at picking out the old. I’ll bet a mature wolf can spot an old deer in a heartbeat. And an experienced cowboy has no trouble identifying an old horse even at a distance.