Abhimanyu was sixteen.
On the thirteenth day of the war Droṇa drew the army up as a cakravyūha — a formation coiled like a wheel, ring inside ring, that closes behind whatever comes in. Very few men alive knew how to open it. Arjuna was one of them and Arjuna had been drawn away to the other end of the field on purpose.
So they asked his son.
And here is what he told them, before he went, plainly and to their faces: I know how to get in. I do not know how to get out.
The story everyone tells is that he heard it before he was born — his father explaining it to his mother, and his mother falling asleep partway through, so he got the going in and never got the coming out.
They sent him anyway. He would open the gap and they would come through it behind him. That was the arrangement and everybody agreed to it.
He went in. And behind him, one man — Jayadratha — stood in the gap and held it shut, and the rest of them could not get past.
He was alone in there for a long time, and he was extraordinary. The book spends chapter after chapter on it and does not hurry.
Then six of them came at him at once. His bowstring was cut from behind. His chariot went, his horses went, his driver went, and he pulled a wheel off the wreck and fought on with that.
And then somebody hit him from behind with a mace, and that was the end of it.
Every adult in that field knew he only half knew it. He told them so himself. They sent him in, and then the door shut, and none of them could reach him.