When children are ready to write
Handwriting sits on top of fine-motor control, and that control arrives on its own schedule. Before a child can form letters comfortably, they need to be able to hold a pencil without a white-knuckle grip and copy simple shapes like circles, crosses and lines. Most children reach that around five to six, though the range is wide and entirely normal.
Pushing letters before the hand is ready is the most common early mistake, because it trades a few weeks of apparent progress for habits that take much longer to undo. Time spent on pre-writing play, drawing, tracing paths, big loopy scribbles on a whiteboard, is not a delay. It is building the exact control that makes letters easy when they come.
A relaxed tripod grip, thumb and two fingers, comes before letters. A tense or fisted grip tires the hand fast and caps how neat writing can get, so it is worth settling early.
Large movements on a whiteboard or in the air come before small ones on a line. Control develops from the shoulder inward to the fingers, so start big and shrink gradually.
Circles, lines, crosses and curves are the strokes every letter is built from. A child who can copy those confidently is ready for letters; one who cannot is not, yet.
Mixing up b and d, or writing letters backwards, is ordinary up to around age seven. Correct it calmly by re-teaching the letter, not by making it a big deal.