Well there is not one, but I do have a personal theory. Just a little note of caution, if your pediatrician does not have extensive experience in international adoption they will think your child is healthy and completely on track with their development as long as they are in the size range of the age that the adoption records indicate. The problem is what if the adoption records are wrong? I posted a long time ago about this issue with our daughter from Ethiopia. Our doc at the time thought she looked great and was exactly the age that we were told she was. Well, he was completely wrong. After multiple developmental evaluations, a burst in growth from good nutrition, and exams from the dentist we learned that she was at least 2 years older then we were told. I am glad I questioned it and did my research and did not back down, because she is now a happy well adjusted kindergartner who looks, acts, and thinks just like a typical 6 year old child. I cannot imagine if we had trusted the doctor, she would still be in preschool and would be towering over her peers and most likely extremely frustrated socially and intellectually. It is any wonder that early puberty is a phenomenon in internationally adopted girls?
So back to my original point of this post. Based on my own observations of each of my adopted children upon coming home and the results of the developmental evaluations that they received, the best predictor of their actual age was their fine motor skills. Fine motor skills are things like the ability to hold a pencil, unlock a child safety lock, pick up small objects and so forth. Now you would initially think that this would be a delayed area of development in a child who has lived in an orphanage but if you think about it it is not an area of development highly affected my malnutrition like physical size and gross motor skills are. Also even if a child is stuck to a physically confined space they most likely are going to explore things with there hands. Also children living in orphanages have to develop adaptive skills to care for themselves and this is where fine motor skills can develop while other areas may be delayed. So I would highly recommend that if you are trying to evaluate your child's age to meet with an occupational therapist. This is the specialist who assesses fine motor skills (as well as sensory issues, an area that your child probably will have some concerns in) and there are tests that can be done at a very young age to determine their standardized as well as comparative age when looking at what is the typical ability level of same age peers. An evaluation from an experienced OT can be obtained through any outpatient hospital OT department, an internationally adoption clinic, or you can receive a free developmental screening through your local school district.
One test that I do not trust because it was wrong for our daughter is a Bone-Age Scan. This is the test that your doctor will recommend to you and it is when they take a picture of the child's wrist and hand. The problem is that the test does not take into consideration the impact of long-term malnutrition on growth and there is plenty of evidence to show that malnutrition will in fact cause delayed growth and in some cases the child will even display a growth hormone deficiency (which our little guy has tested positive for). Also the standard deviation for the bone-age scan is rather large, so no you cannot pinpoint the exact age of your child from this test (regardless of what the doctor tells you). Here is the truth about bone-age scans:
The standards in the Greenish and Pyle Atlas are derived from a study of healthy white middle-class children in the Cleveland area in the United States in the years 1931 to 1942. The study consisted of 6,879 hand radiographs from boys and girls. The ages ranged from 3 months to 16 years for girls and to 17 years for boys at the time of the radiographs. In the Greulich and Pyle Atlas a table is used to provide means and standard deviations for skeletal age1. At the time of its development Greulich and Pyle did not consider the estimation of chronological age as a potential use of their data and Atlas. Their original work was used to identify possible growth disorders, malformations and bone abnormalities.
This info came from a good article about determining chronological age in internationally adopted children written by Cec Pederson, 2004. You can access it on the web at http://eprints.usq.edu.au/3839/1/Pedersen_Chronological_age_determination.pdf