I've been running our kids and teens programs for 5 years and what I've found to work over that time is that it's better to go by experience more than age with the younger ones.
For the younger kids (below the age of 7) , around the time they earn their 2nd stripe on their white belt, I introduce a few basic "safer" submissions. I don't teach any bent arm locks like kimuras or americanas until they're at least 9. The same thing goes for guillotines, as a bad guillotine is a good neck crank.
For our teens, I'll teach them submissions from day one. Bear in mind, that for the first few weeks (maybe even a month depending on the kid), the teens are not allowed to use submissions when rolling. Usually, I'll pair them up with one of our more advanced students and have them work out of bad positions or try to maintain good ones.
For all of our students, the concentration early on is on defending and escaping from bad positions, submissions, and strikes early on; mainly stuff they're likely to encounter on school yards, etc.