To go along with the age limit, you can trade at a younger age as long as somebody signs for you.
Also, brokerages have minimum limits for starting accounts. I know that Scottade is one of the lowest, I believe they have a $500 minimum to start an account which is pretty good for somebody that's wanting to start with little money. They also have $7 commissions which is much cheaper than other outfits.
I'm not trying to advertise for them, but I thought I would throw the idea out for anybody that's interested. Go to their website (or a branch office) to find out their details.
Shop around, you'll find that every outfit has it's pros and cons, some are much more expensive than others, some provide better services than others, etc..