I've heard that the youngest possible starting age is 12, and that's only if you choose to be an orphan to gain playtime at the cost of family bonuses. I'm assuming you can start doing just about anything from the get-go, since wandering around an orphanage would be no fun, and there's nothing quite like a good adventure to kick off your life.
Now the early game is usually one of the first design areas to be tackled, so this has definitely been answered before. But the search function isn't turning anything up, and I don't have the time to read through all of the developer journals one by one, so if anyone knows which ones go into detail on this, please do tell.
Thanks in advance!