You are correct re the diploma. Once you graduate, you must complete the Diploma in Legal Practice. It is a compulsory requirement before you are admitted as a solicitor. Once you do this, you then move on to do your traineeship. However, bear in mind that most legal firms recruit trainees 2 years in advance. I think the figures you give for trainee salary are a tad overkill. Perhaps for a very very large successful firm. The Law Society average for a first year trainee is between £15000 and £17000.
Your age should be no disadvantage, indeed there are laws in force which mean that you cannot be discriminated against on the basis of your age. If anything, you should be able to demonstrate more life experience being older, and this will come in especially useful for any competency based questions in interviews.