For a bone age test what does "standard deviation of 9 months mean? is it 9 months above the mean or 9 months below the mean?

The standard deviation is a measure of how the bell shaped curve of data from say many folks of your age and gender and ethnicity. 2 SD PLUS Or minus includes about 95% of folks studied. By convention if your value IS plus 2 SDs you are high. If below then you are 2 low. Review standard deviation on wiki for more details. You did not give your value it’s self.

Standard deviation (SD) is a statistical term. Mean is the average, Mean plus and minus one SD covers about 65% of the population. Mean plus minus 2 SD covers about 95% of the population. Visit this for more info on this topic: https://en.wikipedia.org/wiki/Standard_deviation