The criteria for putting a unit (either a family or a household) into the classification is the age of the youngest child in that unit.
Age of youngest child is a flat classification with 51 single year categories ranging from one to 49 years plus a category for '50 years and over' and a category for 'less than one year'.
The criteria for putting a unit (either a family or a household) into the classification is the age of the youngest dependent child (under 18) in that unit.
Age of youngest dependent child is a flat classification with 18 single year categories ranging from one to 17 years plus a category for 'less than one year'.
You can download the full classifications from the 'Available files' section.