Around page 2829 of the CE Sanjaya says
O king! Abhimanyu was still a child and had not yet attained youth. But he was killed
He was full of ecstatic praise for Abhimanyu's valor when he was killing Dhritarashtra's soldiers and kinsmen- but Abhimanyu became a child for Sanjaya after he was killed in a one-on-one combat with another soldier who might have been a child too - Duhshasana's son.
Was there an agreement not to use children in the war?
And whether there was an agreement or not, was anybody under age 18 known to have fought in the war?