你倒是想！或者做梦吧！

做梦吧! It actually means something more along the lines of, "Keep dreaming," but that's the closest connotation to "You wish" I could think of.

My mind says I'm in my twenties, my body says yeah, you wish! Here, " you wish" means in Chinese:你倒是想呢。中文里面，它实际表达的意思是否定的含义。 In Chinese, this sentence expresses the negative meanings. That is to say, when you use you wish at a context which the content is not in accordance, then " you wish" means 你倒是想呢。or we can say in Chinese: 这也就是你所希望的吧

我会说：你想得美！/ 你妄想！

My mind says I'm in my twenties, my body says yeah, you wish! 我脑子想我现在二十岁，我身体却说“是吗？你想得美！” （你身体知道你已经不是二十了）