diff --git a/yt/frontends/ramses/fields.py b/yt/frontends/ramses/fields.py
index 6937da007a4..65e4b36acfe 100644
--- a/yt/frontends/ramses/fields.py
+++ b/yt/frontends/ramses/fields.py
@@ -189,12 +189,11 @@ def star_age_from_physical_cosmo(field, data):
             H0 = float(
                 data.ds.quan(data.ds.hubble_constant * 100, "km/s/Mpc").to("1/Gyr")
             )
-            times = data[ptype, "particle_birth_time"].value
-            time_tot = t_frw(data.ds, 0) * H0
-            birth_time = ((time_tot + times) / H0,)
-            return data.ds.apply_units(
-                data.ds.current_time.to("Gyr") - birth_time, "Gyr"
-            )
+            times = data[ptype, "conformal_birth_time"].value
+            time_tot = float(t_frw(data.ds, 0) * H0)
+            birth_time = (time_tot + times) / H0
+            t_out = float(data.ds.current_time.to("Gyr"))
+            return data.apply_units(t_out - birth_time, "Gyr")
 
         def star_age(field, data):
             formation_time = data[ptype, "particle_birth_time"]
diff --git a/yt/utilities/lib/cosmology_time.pyx b/yt/utilities/lib/cosmology_time.pyx
index 90dbe87b71b..0a2eb9d0ec7 100644
--- a/yt/utilities/lib/cosmology_time.pyx
+++ b/yt/utilities/lib/cosmology_time.pyx
@@ -49,13 +49,12 @@ def t_frw(ds, z):
     om_l = ds.omega_lambda
     conv = ds.quan(0.01, "Mpc/km*s").to("Gyr")
 
-    if isinstance(z, float):
+    if isinstance(z, (int, float)):
         return ds.quan(
             quad(_a_dot_recip, 0, aexp, args=(h0, om_m, om_l))[0],
             units=conv,
         )
 
-
     return ds.arr(
         [quad(_a_dot_recip, 0, a, args=(h0, om_m, om_l))[0] for a in aexp],
         units=conv,
@@ -69,7 +68,7 @@ def tau_frw(ds, z):
     om_m = ds.omega_matter
     om_l = ds.omega_lambda
 
-    if isinstance(z, float):
+    if isinstance(z, (int, float)):
         return quad(_da_dtau_recip, 1, aexp, args=(h0, om_m, om_l))[0]
 
     return np.asarray(