"We" as adults at least have the mostly-capable ability to discern good from bad, whereas a child until the age of 14-18 (depending on the topic at hand) does not have the sense of responsibility, especially about the crucial aspect which is his/her own health. Therefore you could expect an adult to be able to take care of him/her-self after the ~age of adulthood, whereas a children is very rarely expected to do so, because most of the times they're not even fully aware of it (consciously speaking, not talking about almost instinctive reactions).