Mm yeah, I find it that the characters are making it weird by making it such a big deal. Both Shaayu and the Demon King already said she was born as an adult, with an adult mind and body. She just hasn't experienced much of the world.
It would be similar if there was someone who was actually the age of an adult, who was confined in one space for all their life before being rescued/getting out. They would be just as ignorant to the things of the world, if not more :/ it's all about how the characters act too, imo Shaayu doesn't act like a child and I don't see why she should be treated like one.
The 5 year old part was completely unnecessary in my opinion
Because other that shock value, it adds absolutely nothing to the story till now, and well it makes the romance very weird-