[zdragnar]

I guess that was my point in a roundabout way- they lie about their age to pretend to be under the legal adulthood age limit.

Some animals do have fairly reliable (if invasive to utilize) means of telling their age- I think their tusks grow rings like trees, which could be counted to measure years.

Humans don't really have any equivalent that I'm aware of. There are approximations such as bone fusing and cartilage density that can tell if a person is in the neighborhood of 18 years old, but I don't think it is sufficient for legal purposes.

For further reading, https://pmc.ncbi.nlm.nih.gov/articles/PMC2465138/#!po=3.3333... discusses using MRIs to determine age, with a roughly 1% false measurement rate on a very specific set of individuals with confounding factors- ethnic differences, nutritional habits, physical activity etc.