I believe mirrors are too complex for a very young child. Simplicity and honesty should be consistently maintained in all aspects of childrearing. A mirror is a deceptive reflection of reality and is confusing to a young child who does not understand that it is a reflection, not a real person. I remember mirror tests done at a university where a large standing mirror was placed in a room. Each child who entered the room looked at the mirror and appeared puzzled, then went behind the mirror to look for the child she saw.
Reality is complex.
A child reaches to touch the person in the mirror and instead touches hard, cold glass or can't find the child behind it. Or she may run straight into it, thinking she is approaching another child. A mirror is fine for an older child who understands what it is.