The Frightful Presence ability description says:
Frightful Presence (Ex)
A young adult or older dragon can unsettle foes with its mere presence. The ability takes effect automatically whenever the dragon attacks, charges, or flies overhead. Creatures within a radius of 30 feet × the dragon’s age category are subject to the effect if they have fewer HD than the dragon. A potentially affected creature that succeeds on a Will save (DC 10 + ½ dragon’s HD + dragon’s Cha modifier) remains immune to that dragon’s frightful presence for 24 hours. On a failure, creatures with 4 or less HD become panicked for 4d6 rounds and those with 5 or more HD become shaken for 4d6 rounds. Dragons ignore the frightful presence of other dragons.
Dragons have age categories ranging from a wyrmling to the great wyrm. What number should be used to calculate the range of Frightful Presence ability?