There is no uniform entry age to secondary (or senior) schools but they usually take pupils up to the end of the sixth form at 18. Broadly speaking, the normal entry is at age 11 (Year 7), sometimes with further pupils coming in at 13 (Year 9). Many traditional boys’ and co-educational public schools start at 13 (Year 9). In Scotland secondary schooling begins at age 12.
Junior and prep schools take pupils up to age 11 or 13, depending on the schools that most of their pupils go to afterwards. If they all go on to senior schools that recruit at 13, they will prepare them for Common Entrance (CE) at 13 and there may not be adequate preparation for any pupils taking entrance exams at age 11. Conversely, if they all go to schools that recruit at 11, there may not be any provision for 12 or 13-year-olds at all. You need to make sure your overall plans are in synch.