My eldest began writing like a fiend at age 8.5. It was like her cork came unstopped and it just poured out of her. She's an incredibly gifted writer. Creative fiction was her first passion, but poetry and eventually persuasive essay writing, reviews and editorials as well. She mostly uses a computer, but she developed decent handwriting around age 9.
My ds didn't begin writing until age 10 or 11, and then it was only on the computer, and rarely fiction, mostly computer game reviews, walk-throughs and such. He has dysgraphia, so handwriting never really worked for him: at age 16 his "signature" is still a tediously slow manuscript. But in the past couple of years he's become a highly entertaining deep thinker and expostulator. He keeps up several long-distance friendships by email. He has several blogs where he posts about things like man's basic nature, atheism, the war on drugs, true friendship, etc. etc.. He's active in various debate reddits and writes with great maturity. His social studies teacher used one of his blog posts (with his permission) as one of the assigned readings for a class exploring models of social justice.
My middle dd developed decent handwriting gradually around age 8-12. She isn't a huge writer, but she is certainly at least "at grade level" and is having no trouble now that she's in high school getting good grades in advanced English and Writing courses. Her handwriting is fine; not too fast, but quite legible and sufficient for her needs. She uses a laptop for a lot of her schoolwork, but unlike her brother she doesn't need it to cope; she just likes being able to edit and print tidy copy.
My youngest is now 9. She writes well, but not a lot. She hasn't completely mastered cursive, but has a nice hybrid style that she's happy with for now. Compositionally the quality of her occasional bits of writing is excellent; on a computer her spelling, grammar and vocabulary make her writing seem like the work of someone much older.
So on average my kids developed their handwriting gradually between ages 8 and 12 (except for ds). For the most part they only began doing a lot of writing only once they became much more independent and their worlds expanded: then they began to need to write in order to communicate with friends and explore ideas and make contacts out in the big wide world. That urge kicked in around adolescence. Even though my kids wrote almost nothing before age 8, and very little before age 12, they have totally excelled in the creative writing courses that our public school offers. The later start didn't seem to produce any lags or difficulties.
Kids at school have to produce written work to facilitate the evaluation process. Conversation doesn't work well to gauge the mastery of 20 or more individual children in a classroom, so teachers have put tremendous emphasis on the early mastery of writing. It may be no big deal for many kids, but for others it can feel tedious, even painful, and interfere with their love of learning. I appreciate the way unschooling has let my kids come to writing in their own ways, on their own time.