about 1908-10
I just have the idea to count the number of pearls of the necklaces; for Olga the number is about 30 and Anastasia 20.
SO the girls are about 15,13,11 and 9 year-old...
BUT Olga's hair is not yet up...
Here are 2 photos of Olga in summer 1911 and in november 1911 (her 16th birthday) to compare. On the firts her hair is not totaly up, ond the second, it is.
SO the year can't be 1912 for the Borodino celebrations: below, Olga and her uncle Mikhaïl in Moscow foe the Borodino celebrations: