I think that you can start letting them try drawing (eg., on a doodle board) as soon as possible. The key is to develop the fine motor skills or the fingers and hand, so drawing is a good (not to mention fun!) way to start. I think they can be scribbling and doodling soon after 12 months, if not sooner.
Next would be to get them to draw in constrained ways, eg., join dots, draw along (faint) lines in differing patterns, etc.
So as for writing, I think you can start whenever you feel your child can control their drawing movements sufficiently. You would then get them to 'draw' out the letters of the alphabet in the same way as you would get them to follow drawing patterns.
Felicity has been writing for quite a few months now - wrote me her first Father's Day last Father's day! (with Mommy's supervision, of course!
).