Lance Platt is an expert on standardized field sobriety tests in operating while intoxicated investigations and recently presented here in Indiana. Two interesting tidbits:
First, the “walk-and-turn” and “one-leg stand” tests have not been normed to any particular age group. So, an older person who is losing his/her ability to balance is more likely to struggle with the tests than he/she would have when he/she was younger. That was demonstrated when folks between the ages of 20-80 tried the tests. The older the person, the more he/she struggled. The same is likely true for teenagers going through their growth spurts where they have all of the coordination of Bambi crossing the icy pond.
Second, if there ever was a head injury, the ability to do the balance tests could be compromised long-term, as the vestibular system in the inner ear that contributes to balance may be impacted in unknown ways.
The handbook on the standardized field sobriety tests is available from many sources, including here. The student manual is the one that the police officers use during training. Dr. Platt’s curriculum vitae cites many publications concerning the three tests, with a particular focus on the horizontal gaze nystagmus.