How old is White Fang when Gray Beaver makes another great trip north?

White Fang was five-years-old when he made the trip North with Grey Beaver.

When White Fang was nearly five years old, Grey Beaver took him on another great journey, and long remembered was the havoc he worked amongst the dogs of the many villages along the Mackenzie, across the Rockies, and down the Porcupine to the Yukon.