"you have to be 21 to hold a licence, and it takes 3 to 5 years depending on training and experience."
Duff gen. Part 66.A.15 states:
"An applicant for an aircraft licence shall be at least 18 years of age"
Part 145.A.35(m) states:
"The minimum age for certifiying staff and category B1 and B2 support staff is 21 years"
Quick way through would be to enlist in an ab initio part 147 basic training course (2400 training hours of which at least half must be theory) but you would still need 2 years post course experience to get the licence issued. Another option is to carry on with the apprentiship and enroll on a distance learning course from a part 147 college. I should point out that I have no idea of the cost of the course.