Fun fact: frontal lobe development is usually completed around age 25.
Per canon, Schmendrick is cursed to be immortal at age 19 (explicitly said in the short story "Schmendrick Alone"), so since he has not physically aged since then, basically it is also canon that his brain has not completely matured.
One can argue about the life experiences he has had in the meantime, but I cannot stop laughing about Molly bringing up the issue to quickly win an argument.
(But then again, she has ended up with a partner who has the body and stamina of a 20yo very likely paired with the experience of a grown man, so she does not complain much in the end)













