What is the maximum sized cc motorcycle you can ride with a cbt in the UK at the age of 26yrs old?

An engine up to 125 cc with a power output not exceeding 11 kW and you need 'L's . You might want to check the position on a small classic british motorcycle around 200cc, none of them had a power output of more than 11kW and were legitimate in the early days of the law.