In Oxford can take any person — regardless of age. However, due to the high competition and the difference between the British and Russian programs, it is better to start preparing as early as possible.
If You are going to enroll in a British College, in 12-13 years you should start to explore options to tighten the language and keep track of progress.
Well, if the child happens to go to school with a good reputation. The surest way to prepare for such school will be the passage of a special summer course at one of them. Known cases when graduates of elite schools were able to go to Oxford even without the brilliant evaluations.
Despite the fact that the prestige of the school still is of great importance when considering the application to the University, it does not mean that attending such schools is the determining factor.