diff --git a/c3l_membership/utils.py b/c3l_membership/utils.py
--- a/c3l_membership/utils.py
+++ b/c3l_membership/utils.py
@@ -1,7 +1,7 @@
 from datetime import date
 
 
-def calculate_age(birthday):
+def calculate_age(birthday: date) -> int:
     today = date.today()
     born = birthday
     try: