Kazoedoshi (数え年) is a traditional way of counting age used in Japan (and in other parts of East Asia).
When you are born, your age is counted as 1 year old, and you get 1 year older on every January 1.

How to find your Kazoedoshi?

When using your Western age, you can try to calculate it like this:
At the start of the year, before your birthday, you add 2 years. After your birthday, you only add 1 year to your Western age.

You can also find a Kazoedoshi calculator here.

In Shinto, age-related ceremonies usually use Eastern age, so it is useful to know it.
Some of the ceremonies using Kazoedoshi are Nagoshi-no-Harae and Toshikoshi-no-Ōharae.

On Setsubun, people in Yakudoshi (years of misfortune) should request a Yakubarai Gokitō ceremony to receive blessings to ward off evil and to cleanse themselves of evil influences, based on their Kazoedoshi age.