My friend Leigha's little daughter Sydnee writes in mirror-image. Thought I'd look into it. Here's something I found online at the URL above:
Five-Year-Old Writes Backwards
Pediatrics Expert Advice from Shari Nethersole, M.D.
Question: My five-year-old granddaughter, when trying her skills at writing, usually starts on the right side of the paper rather that the left. About half of the time she mixes her Bs and Ds and makes her S backwards. She has been in a structured day care and is getting ready for kindergarten this fall. Any need for concern, especially about dyslexia?
Answer: There is no need for concern if your granddaughter is otherwise well. It is completely normal for children to write "backwards" at this age. In addition to letter and number reversals, some children will truly write in mirror image: going from right to left with all the letters reversed. There is nothing wrong with this. The brain does not completely form the concept of left and right until somewhere between ages five and eight. This means that almost all children will have persistent reversals when they first start writing.
meop rof eendyS
t'nod eb eulb
t'nod og srekcarc
s'ti yako fi er'uoy a elttil sdrawkcab
sgniht teg ssel erup sa uoy worg dlo
od ew lla trats sdrawrof, neht og
sdrawkcab nehw er'ew dlot?