I have never heard of any blood or tissue test that would give a person's exact age, and I doubt that such a test would be possible. Xrays of the hands and or feet give reasonable estimates of age in children. Various bones become calcified at reasonably consistent ages. You can use www.google.com to search on the term "bone age". Above age 15, the use of Xrays of joints and bones might be able to give a very rough estimate of age, say plus or minus 10-20 years.

Looking at someone will also let you determine skin wrinkle formation. I seem to remember carnie's being able to come within 3 years fairly regularly even without Xrays. They probably have more skill at age guessing than doctors.