I thought similarly, or that it was perhaps 16-18 year olds taking it upon themselves for a science project or something. However I'm now assuming it is people in their mid-twenties who apparently regard themselves so free of the inherent responsibilities of life that they refer to themselves as kids.

I'm 22 and married. I immigrated to Canada (and the legal responsibilities inherent in this would likely scare the life out of these two 'kids'). My friends and family members of similar age to us have children (in fact our marriage is directly responsible for one of them), and I will likely have children before the two authors consider themselves 'adults'. I have responsibilities and obligations, like I assume most twenty year olds do, and I consider myself to have very few responsibilities in comparison to many of my friends and relatives.

It must be a privileged life to be able to consider yourself a kid well into your twenties, I stopped thinking of myself as a kid when I had to get a full time job at 16. I definitely stopped thinking of myself as a kid when I was handling an intercontinental relationship and funding international travel, which as I said before has resulted in me immigrating to another country.

I think the words "kids" seemed to come from the fun and informal style of writing that we tried to use.