Hey, good to see you here. I would say any age is good as long as the child is interested and the teacher knows the limits of the child. REal interest and the desire to excel will start only after 10, in most children. If you have a child-friendly place near your home I would think even 3 is not too young - We are actually looking for our three year old, but want to be absolutely sure the teacher there knows how to handle little ones and does not get frustrated if the students are not doing what she/he wants them to do.
second tikka here. The teacher must know to handle kids. My niece went to dance class at 3.5 yrs, but the teacher was very strict and she didn't like it. But she is great at home where she dances for every song on the TV