How can you tell a slug's age?

Answer 1

You can check it's ID.

I'm not aware of any reliable field test for age. Size is not simply a function of age contrary to popular opinion.

Some of the older salamanders I have seen have skin that looks "puffy", but that could be a condition not related to age.

Answer 2

size;

for every inch that is one year. worms only live to be 2 years max.

Answer 3

There is no true way to tell a slug's age without dissecting the slug. If a slug can reproduce, it must be at least 1 month old.