It could also be due to a scientific experiment gone wrong. Some mad scientist was trying to mess with time or genetics and accidentally turned the boy into a baby. For example, if they were working on reversing the aging process but didn't have the proper controls in place.
One possible reason could be a magic spell. Maybe a wizard or a witch cast a spell on the boy for some reason, like revenge or as part of a test.
Some kind of scientific experiment error. For example, if there was research on hormonal or genetic manipulation and something went wrong during the process.
Magic gone wrong is a common reason. Just like a magician who miscalculates a spell and accidentally turns someone into a baby boy. Another reason could be a curse. For example, if someone angers an evil witch, she might put a curse on that person to turn them into a baby boy as a form of punishment.
Magic could be one reason. Like in a fairy - tale world, a magic spell might be cast on the boy. Another reason could be a scientific experiment gone wrong. Maybe some sort of genetic - altering experiment that was supposed to do something else but instead turned the boy into a girl.
A scientific experiment gone wrong. If he was in a science - fiction type of setting, perhaps some crazy scientist was testing a new device or formula that had unexpected side effects of gender transformation.
One reason could be revenge. Someone might want to take revenge on the boy or his family by changing his gender. Another reason could be a crazy experiment. Some mad scientist - like character in a story might think it's an interesting experiment to see how a boy would adapt to being a girl.
Maybe the boy has some health issues in the past, so his family got used to taking care of him like a baby all the time. For example, if he had a long - term illness when he was younger, his parents might continue this way of care even after he recovered.
Magic potions. In fairy - tales, magic potions often cause strange transformations. A boy might accidentally drink a potion meant for something else and turn into a girl.
Magic could be a reason. Maybe a wizard or a witch cast a spell on the boy accidentally or on purpose. Another possibility is a strange scientific experiment gone wrong in a fictional world. For example, if some mad scientist was trying to test a new gender - changing device.
In some turned into a girl stories, it could be due to a curse. For example, if someone offended an old witch or a powerful spirit, they might be cursed to change into a girl. Also, there could be a strange natural phenomenon. Like in a place full of strange energy, a person could be transformed into a girl just by being there for a while.
Curses are also common in such stories. An angry sorcerer or an evil spirit might curse a boy out of spite or revenge. The curse could be designed to change his personality and appearance in a way that makes him seem sissy, and of course, the boy would be unwilling as it's not his choice.