Russian boys seem to have worn sailor suits a lot later than boys elsewhere. There is a photograph in John van dear Kiste's biography of Xenia of Xenia's eldest son Andrei on horseback wearing a sailor suit. According to the caption, this was taken in spring 1914. Andrei was then 16-17.
There is also a photograph of Alexander III shortly before his death with Mikhail, then approaching 16 and wearing a sailor suit.
Ann