What a twist - it's interesting! But Olga and Tatiana were both rather 13 and more than a half year in July of 1909 and January of 1911. (both of them were given there regiment approx. 4 months before their 14th birthday). I'd say that 13 -13, 5 years were 'the lower limit'(leaning towards 14 yrs). for getting their regiments (OTM). Also, the difference between 1909/11/13/15 is 2 years, same as the sisters' ages differences, and all four got them when they were at last 13.