When you have your first litter of pups, your wolf will be a 3 year old wolf, and then you will can choose Age Perks.
Advancing in age is not obligatory, if you want, you can stay at a certain age and then your wolf will not age anymore.
If you choose to age up, you will can select a maximum of 3 Perks.
Perks are classified in 3 types: Physical Attribute, Clout and Wisdom.
In every Age has at least 1 different Perk.
At Age 8 your wolf will be an Elder Wolf that anytime can die. So if you choose to advance to age 8, be sure that your wolf can now die at anytime and when dead will not be playable anymore.
See More about age Perks here.