I would err on the side of caution with this one. at time of shooting that model could not have approved those images, it's an interesting question: do they now have adult control over their past images? will my own children, upon turning 18, demand all <18yo images of them at various sites be removed? if they can sign for their junior self then surely they can also question the signed release?
I would say they can't sign for their <18 self. but if you got away with it, and you think there's no drama in the future, then go for it.