@forest You're perhaps thinking of it as one child choosing to hang out with a somewhat older child. However, a 17 year old developmentally is essentially an adult, not a child by any stretch. So you have a situation where an non-related adult, is choosing to hang out with a young child. Consider then, what if this older person was not 17, but 55? Would you still be perfectly ok with a random 55 year old adult wanting to hang out with a 9 year old?
@BCLC The difference between social hanging out and babysitting is in the motivation behind it. For babysitting, the babysitter is usually doing it for payment, or as a favour etc. They're not doing it because they just want to "hang out" with young kids. Since it is so unusual for a 17 year old to seek out the company of young children, it's reasonable to be highly suspicious of their motives.