It’s completely normal for children to write “backward” as late as age 8. 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.
The brain doesn’t completely form the concept of left and right until somewhere between ages five and eight. That means almost all children will have persistent reversals when they first start writing.
Don’t stop your child from writing this way, and don’t make your child correct it every time, especially in Early Childhood. If your child asks if it’s correct, you can point out the errors, but don’t make a big deal of it. You don’t want to limit your child’s progress by constantly pointing out what’s wrong and frustrating him or her.
The more children write, the easier it becomes for them. When your child learned to speak, you applauded all of your child’s efforts, even if “Dada” didn’t quite sound like Dad. This is how your child kept trying and kept improving.
Similarly, with writing, children should be free to practice and make mistakes. Celebrate that they want to write and how you can start recognizing the correct form of a letter, even if the letter is reversed. In school, teachers will ask children to correct their reversals, and by the end of second grade almost all children have stopped doing it completely.