Was Samantha Older Than Everyone Believes?
The below needs some further looking into, I am giving here only the information I have received, without making any conclusions.
In Zenowich's documentary Roman Polanski: Wanted and Desired Miss Gailey's school ID is shown full screen (I'll post the picture later), and it can be clearly seen that in 1976/77 school year she was in the 9th grade; as far as I know (though I may be mistaken), the age for this grade was 14-15 years old, not 13. I haven't found any indications that she skipped a grade at any time. A reliable source sent me this record:
according to which she would have been 14 and a half on March 10, 1977, not thirteen. It would explain her school grade, and - at least to some extent - her remarkable maturity. The Grand Jury transcript specifies that only a copy of her birth certificate was submitted by the family, not the original. If we remember that the family tried to falsify evidence (see this chapter), I wouldn't be very surprised they lied about her age, too. It has also been noticed by many that the information on her age at various periods, as given in interviews and in different sites, is often conflicting - but this is a matter of further research.