Well, it varies from person to person. My sister, who is younger than me (she's 24) still lives with my mom. They keep each other company and she helps out with the chores and bills.
As long as she's helping out it's ok. I mean it's better than being alone and if your parents want you to be with them it should be Ok. For instance, if my daughter told me she wanted to live with me, I would always have the door open for her.