In days gone by the thinking was that the age of a child dictated how many kms they could walk per day.
Most kids in Oz don't run around outside as much as we did when we were small , we didn't have a screen of some sort in front of us the whole time and on the whole we were more active and not as rotund as many kids are today. Bushwalking /trekking means I can still hike rings around most people aged 6-60 in terms of stamina .