Remember to:

RIGHT: 5-year-old, 10-minute, 9-kilometer, 7-month, 3-storey, 2-storey, 3-room, 256 keyboard.

Disgusting: 5 year old and so on.

Why Why? Because it
Five years old,
10 minutes of clutter;
"five" cut to 5 (five with a defect) and glue the following root: 5-year-old.