It depends entirely on your daughter. You are really the BEST person to make that decision, since you know her best. Things to think about, though, are can she...
- Listen to a whole story without getting distracted
- Cooperate with other children
- Recognize that letters represent sounds
- Speak so everyone can understand what she's saying
- Follow basic instructions and rules
- Use the bathroom
- Handle the social situation of a classroom (probably if she's in preschool)
- Count to 10
- Knows her colors/shapes
- Sort objects into categories
And most of all... does she WANT to go, yet? At barely 5, she should be old enough to have some sort of idea if she wants to keep doing what she's doing, or start on the next adventure in life! Talk to her about it!