Actually, 牠，它 and 祂 can all be used in Simplified Chinese, they're all still ㄊㄚ/Tā (I double checked on my keyboard) but it's kind of like 妳，it can be used in both but Chinese people prefer to just use “你”
Because that would be saying that they are "a six year old" as if you're an object called "six years old"
Like if you said "我是一隻貓” (I am a cat) you would be saying that you are physically a cat.
Sorry, my explination is bad but I tried. I hope that helps.