They were just children when their fathers ran toward the twin towers on Sept. 11, 2001. They grew up revering parents — firefighters and police officers — who were killed that day, or died years later from the toxic dust.

When a reporter starts to ask them “How old?” — wanting to know their current age — many reflexively answer “7” or “5” or “10,” their age when their families were upended by a terrorist attack that remains painfully etched in the city’s collective memory.

On Tuesday, a record number of these children of slain rescuers will take an oath, like their fathers did, to serve New York City.

Of the class of 301 trainees graduating as probationary firefighters, 21 are children of men who died in the line of duty. Their ranks include 12 sons and one daughter of firefighters killed on Sept. 11; six sons of firefighters or police officers who died of diseases linked to their time on the pile; and the sons of a firefighter and a police officer who died on the job.