By the age of 4 rose greys usually have a lot of white hairs mixed in. They also turn grey at very young ages. If you aren't seeing white hairs, and she has black points, mane, and tail, she'll probably won't turn grey.
It's very rare for a horse to stay like that in fact... She is a 4 year old Andalusian. But her mane and tail are black and so are her legs from the knee down. She has bay all over her and on her face and since her dam is dark bay I'm hoping she'll stay like that.