Age 3ish
some kids will happily manage it at 2 if they are given activities to increase their fine motor skills. Things like making small ants from play dough, threading beads, drawing, tweezers, scissors, tearing paper into strips, hand picking up peas for finger food....
My girls wrote their names beautifully at 3 and wrote letters to nana that were entirely readable at the same age. My son doesn't want to write letters to nana
but can print his name and words HE wants to write!
I see no reason to wate until they start school. Once they learn to read early they potentially could get frustrated that they can't write their own stories down. If you wait until 5 or 6 to teach writing they have been reading for maybe 4 years! I also suggest teaching typing at the same time if you can
But it's often boring for them
Some starter ideas. Write and wipe books are great. As are laminated name cards. Also white boards. A good idea to begin with is to use reduced resistents products. It's easier to write on a white board and on paper with textas than to write with pencil and paper.
Tracing lines and swirls can be fun for kids and helps with pencil control. Rememeber when we were encouraged as kids to outline our drawings in texta? Also those pencil grips are especially useful for little kids who don't have good pencil grip. Try to get one with a moulded shape so the fingers automatically go in the right spot. Writing is one of those things that dramatic improvement will occur with consistent practice. So just a few letters a day will probably be better than a page a fortnight.