In the old days our extended families and rural life meant we saw a lot of interactions across all age groups.
We have lost so much of that that I now people saying things like "teens look like children". It's completely absurd but I think they really believe it. They have never seen a 17 year old stand next to a 7 year old. They never think it thru. Like, obviously a 16 year old can pass for 22, but you'd never confuse him or her with a 10 year old. Children look VERY different from teenagers. Puberty does that to a person.