diff --git a/agenda/birthday.py b/agenda/birthday.py
index a847a79..fdebd5d 100644
--- a/agenda/birthday.py
+++ b/agenda/birthday.py
@@ -6,15 +6,20 @@ import yaml
 
 from .types import Event
 
+YEAR_NOT_KNOWN = 1900
 
-def next_birthday(from_date: date, birth_date: date) -> tuple[date, int]:
+
+def next_birthday(from_date: date, birth_date: date) -> tuple[date, int | None]:
     """Calculate the date of the next birthday based on a given birth date."""
     next_birthday_date = birth_date.replace(year=from_date.year)
 
     if from_date > next_birthday_date:
         next_birthday_date = birth_date.replace(year=from_date.year + 1)
 
-    age_at_next_birthday = next_birthday_date.year - birth_date.year
+    if birth_date.year != YEAR_NOT_KNOWN:
+        age_at_next_birthday = next_birthday_date.year - birth_date.year
+    else:
+        age_at_next_birthday = None
 
     return next_birthday_date, age_at_next_birthday
 
@@ -26,14 +31,18 @@ def get_birthdays(from_date: date, filepath: str) -> list[Event]:
         entities = yaml.safe_load(f)
 
     for entity in entities:
-        birthday = date(**entity["birthday"])
+        if "year" in entity["birthday"]:
+            birthday = date(**entity["birthday"])
+        else:
+            birthday = date(year=YEAR_NOT_KNOWN, **entity["birthday"])
         bday, age = next_birthday(from_date, birthday)
         for offset in range(2):
+            display_age = f"aged {age + offset}" if age is not None else "age unknown"
             events.append(
                 Event(
                     date=bday.replace(year=bday.year + offset),
                     name="birthday",
-                    title=f'🎈 {entity["label"]} (aged {age + offset})',
+                    title=f'🎈 {entity["label"]} ({display_age})',
                 )
             )