It's less that adulthood started earlier and more that they had a different view of childhood. Bar and bat mitzvahs still occur today and mean much the same thing. Around that age now we are no longer a child and as a teenager we "should know better". Though the term teenager is recent, that 13-21 age has always been seen as an important bridge between childhood and adulthood. Commonly the starting apprenticeship age was around 13 and lasted 7 years. Today in America students move up to high school around age 14.

Today we view childhood as a period of life where the child should be free to explore, experience, and learn- mostly through play. They shouldn't be tied down to responsibilities or obligations like adults are. So the thought of child labor seems horrifying to us. The middle class evolution in the late 19th century made this carefree childhood possible for so many. There are certainly plenty of horror stories of children working throughout the industrial revolution, as there are similar stories of adults. Children working in the 18th century often meant small, detail work. Sometimes because their mother had to work and had no where to leave them. If she could bring her child with her and have them make money doing easy labor, that could mean a world of difference to a family. Very often work like embroidery, covering thread buttons, or other jobs requiring tiny hands were given to them. Work was also given out to women at home from the shops, such as the fabric uppers of ladies shoes. Children could sew along side of their mother and make money to do what we consider a hobby today.

To those earlier societies, they weren't taking away their childhood by having them work. Learning skills, how to use your hands, and being an important part of the household were all things that would prepare them for life ahead. Just like learning to read, write, cook, clean, etc. It was their version of school.