diff --git a/c3l_membership/views.py b/c3l_membership/views.py
--- a/c3l_membership/views.py
+++ b/c3l_membership/views.py
@@ -8,6 +8,7 @@ from flask_babel import gettext
 from flask_weasyprint import HTML, render_pdf
 
 from .forms import MembershipForm
+from .utils import calculate_age
 
 root_page = Blueprint("root", __name__, url_prefix="/<lang_code>")
 
@@ -53,20 +54,8 @@ def index():
 
     if form.validate_on_submit():
 
-        today = date.today()
         if form.birthday.data:
-            born = form.birthday.data
-            try:
-                birthday = born.replace(year=today.year)
-            except ValueError:
-                birthday = born.replace(year=today.year, month=born.month + 1, day=1)
-
-            age = today.year - born.year
-
-            if birthday > today:
-                age -= 1
-
-            if age < 18:
+            if calculate_age(form.birthday.data) < 18:
                 form.minor_member.data = True
                 form.student.data = True
             else:
@@ -109,6 +98,7 @@ def index():
         elif form.payment.data in ("digicash", "satispay"):
             price = price * 100
 
+        today = date.today()
         xml_data = {
             "status": status,
             "name": form.fullname.data,