A good source to consider is a book called "Rescue of the Innocents" by Ronald Finucane, which examines miracles performed on children throughout the middle ages, and it contains a lot of primary source material on children in the middle ages. There is one account from England in 1307 describing the death of a child by drowning and her miraculous revival--detailing the way the children were left by the parents as the parents went about their own business. In the inquisition of the parents, they state; "[My husband and I] with our many neighbors, went to a beer tavern in Walter de la Wile's house in Marden Village. When we left the house we left behind Joanna our daughter, then not yet five years old. Joanna, however followed us to the pub, where she found John , then slightly younger than herself. As I watched, John and Joanna left the pub together and went into a garden or clearing by the pub, and I did not see what they did there." John eventually pushed Joanna into a pond where she drowned....

Here we can see a clear example of the parents abandoning their children in the town while they went to a pub to drink beer. While this is not representative of every parent in the middle ages, it is a good example of how children were often left by themselves, as communities, especially farming villages were close enough for the children to go about on their own. We can see how this did not end well for them....

My answer would be that as soon as a child could work, either in the fields, as an apprentice, a squire, or as a seamstress, they were viewed as an adult and were given the responsibility as such. Hope this helps.

*I'm a history major

A good video on the subject of Medieval childhood. https://www.youtube.com/watch?v=yrVise5qvx0&list=PLrYzzr8yja6EXGbIsrT_V07Mvj-PZGtRt