I am not an expert in these things but my wifes family have been Freeburgesses (Freemen) since the 17th century.
As with most things in days gone by, the position of Freeburgess was inherited through the Male line of the family. The holder would be ADMITTED on the first COURT day after he attained the age of 21. If there was no Male line then the Family will no longer have a claim. My brother-in-law is the last to hold the position as his family consists of 3 girls.
Another requirement is that the person claiming entitlement has to live within the boundaries of Colchester. I have tried to find records but since an initial success going back to 1650, I have had no additional info. (That was nearly 30 years ago) and I can't find the original document).
As far as apprentiships are concerned I believe that they could start from the age of 15 / 18, and last for, usually 7 years.