I am not sure of the youngest age of the first admissions in 1897, but in 1947 the School's First Headmaster wrote in the Silver Jubilee Edition of the School Magazine:
"The school was started under the Acland scheme of organised science schools. The Birmingham school was, I think, the only one that attempted to fulfil Acland’s purpose: to provide a full secondary school course for boys from 12 to 16 years of age, based upon English, modern languages, science and much practical work, and to provide through or in addition to such a curriculum ethical, cultural and physical training."
So I guess the answer to your question is 12