diff --git a/test/growth/calc/age_test.exs b/test/growth/calc/age_test.exs
new file mode 100644
index 0000000..8cc8760
--- /dev/null
+++ b/test/growth/calc/age_test.exs
@@ -0,0 +1,46 @@
+defmodule Growth.Calc.AgeTest do
+  @moduledoc false
+
+  use ExUnit.Case, async: true
+
+  alias Growth.Calc.Age
+
+  describe "calculate/2" do
+    test "consider today as date of measurement" do
+      date_of_birth = Date.add(Date.utc_today(), -7)
+      assert 7 == Age.calculate(:day, date_of_birth)
+    end
+  end
+
+  describe "calculate/3" do
+    test "calculate age in days" do
+      date_of_measurement = ~D[2024-06-09]
+      days_of_birth = [{-1, 1}, {-7, 7}, {-30, 30}]
+
+      Enum.map(days_of_birth, fn {days_ago, expected_age} ->
+        date_of_birth = Date.add(date_of_measurement, days_ago)
+        assert expected_age == Age.calculate(:day, date_of_birth, date_of_measurement)
+      end)
+    end
+
+    test "calculate age in completed weeks" do
+      date_of_measurement = ~D[2024-06-09]
+      days_of_birth = [{-1, 0}, {-7, 1}, {-16, 2}, {-30, 4}]
+
+      Enum.map(days_of_birth, fn {days_ago, expected_age} ->
+        date_of_birth = Date.add(date_of_measurement, days_ago)
+        assert expected_age == Age.calculate(:week, date_of_birth, date_of_measurement)
+      end)
+    end
+
+    test "calculate age in completed months" do
+      date_of_measurement = ~D[2024-06-09]
+      days_of_birth = [{-1, 0}, {-16, 0}, {-31, 1}, {-70, 2}, {-93, 3}]
+
+      Enum.map(days_of_birth, fn {days_ago, expected_age} ->
+        date_of_birth = Date.add(date_of_measurement, days_ago)
+        assert expected_age == Age.calculate(:month, date_of_birth, date_of_measurement)
+      end)
+    end
+  end
+end