I think 你 stop aging. It wouldn't make sense if 你 age when Time has stopped. So, if 你 use the Time and went back a day, then you'd grow a 日 younger again. That's where 你 can see a big difference between the Time and the Return. When Sakura went back to see Clow Reed, she was still the same age. But she still grows older while she is in the past, because Time still continues on in the present...if 你 get what I mean.