While it is suggesting it was common at the time, it doesn't outright state they're talking about that time. At earlier points in history it certainly was acceptable, but we probably don't have pictures of it to go in textbooks. This reeks of them having a general point to make and having a picture that almost fits that point. I've made more tenuous connections for college papers before.

Also, while it's not as drastic, I was doing some looking into family history recently and I found some ancestors who got married around that time. The marriage certificate listed the wife as 17 and the husband as 21... but the math didn't add up when I found their birth certificates and on the marriage certificate she was aged up from 15 and he was aged down from 22. It was in a small farming community and at that point in time and place schooling was largely abandoned during harvest and as soon as kids were old enough to help out on the farm full time they would just stop with school. And for women, helping out on the farm meant taking care of the house and raising kids generally. Time at school was a waste for them so they just got right to the adult stuff immediately.