OK, I’ll take your word. I thought I’d read in a recent newspaper article that it was 15, but I may be misremembering.
I think partly it has to do with - a “child” is still a child while they are dependent on their parents because they are still pursuing education, which until recently typically ended at 18.
In modern times in Western culture, I presume part of the issue is that - school. At one time, finishing Grade 8 (approx. age 14) was deemed what should be a mandatory education. Then it was grade 10 for those not going to college - age 16. Then an appropriate education, paid for by the state, was set to 18 - grade 12.
Fun fact, Ontario used ot ahve Grade 13, making them different from the real world. Meanwhile, funding for the Catholic School Boards was set in 1867 when Canada was formed, so was limited to up to Grade 10. There was a supreme court ruling that grade 10 was sufficient education for a Catholic. (Then students typically transferred to the non-denominational -i.e. protestant public schools) It was only political expediency that changed that in the 1980’s, while Grade 13 was eliminated as a cost-saving measure.
But it seems assorted ages coincided with education milestones - children could work at 14, at which age they could also drop out of school, become apprentices, etc. (in the good old days). 14 is also now the age of criminal responsibility in Canada, IIRC. The later milestone of 16, grade 10, later became the milestone at which children could leave school, work full time, etc. The age at which someone is fully adult (except for the right to drink alcohol, 19) appears to coincide with the end of formal education; note it is also the age at which several other assumptions are made, for example no longer covered by parents’ health benefits - unless they continue education.