At what age do you stop...

I am trying to help my boyfriend stop sleeping with his daughter. She's 8 years old. At her moms she has her sister in the room. She always has had him sleep with her. I told him she's at the age where she's gotta learn to sleep alone. They only time it'll work is if we say you'll have a reward of you sleep thru the whole night alone. Then she will. I'm tired of always getting up in the middle of the night getting him cause he'll knock out until the morning. What do we do? And how old do they have to be in order to stop?