I don't believe there is a maximum age limit as such, I think you are limited by the age at which you can hold a Class 1 Medical.
I presume the real limiter will be down to common sense i.e. will you be able to work for enough years to make the £100,000 outlay worthwhile. Also, whilst airlines cannot discriminate over age, they would clearly prefer someone who has the potential for command etc as opposed to someone who will complete line training then retire.
Good luck!