Yehudayona: I saw in a sefer of Sheilas and Teshuvas of one of the contemporary Gedolim (either Rav Elyashiv zatzal or R’Shlomo Zalman zatzal or R’ Chaim Kavievsky, shlita, ybd”l) that someone asked him for the source of the “minhag” that women aren’t makpid to daven by zman tefila and just make sure to daven by chatzos. His response was: “I never heard of such a minhag.”
Lmaaseh, minhag haolam is that women are not makpid to daven by zman tefila, but there does not seem to be any real halachic reason for it.
I think it may have to do with the fact that women tend to be more meikel about davening in general since they have more things preventing them from davening combined with the fact that since there is an opinion in the Rishonim that women aren’t chayiv to daven, even though we don’t posken that way, there is what to be somech on b’eis hatzorech.
But there really does not seem to be a good reason for single girls to not be makpid on zman tefila.