Technically it should be 16 if you are going by the legal minimum age for full time work.
However, that does not mean employers are obliged to employ people of that age. An employer can determine any minimum age it wishes up until 18, at which time, as an adult, the argument of discrimination can be used.
I seem to recall reading a previous question like this where the answer was that Apple prefered full time employees to be 18 but junior help could be employed, at the manager's discretion, at 16.
James, there is no harm in stopping by and asking in person, taking a brief resumé of your history to leave for consideration and maybe offering to help running general errands on Saturdays. Remember, client contact requires training and while knowing about computers is one thing, being able to handle the public is quite another.