She is inexperienced and naive because she was raised by a conservative religious group. Taking that into account, 20 is about right.
Besides, what is the "age of consent" in the Sith Empire jurisdiction? Would a DS character even care?
Actually she was raised on a bombed out planet in the middle of hordes of rhackghouls, pirates and an imperial invasion. And she wanders around alone. hard to believe she hasn't seen a lot more at 20 than most people see their whole lives.
personally, I think she was dropped on her head as a child - almost nothing she says or does makes much sense. He Lekku should be floating, their so full of air.