If you are under 18 you can't. Much information will be asked and the lie will show up right away. In almost all cases they will require you to enter a drivers license or state I.D. number. They are coded to show your date of birth. Of course, that comes after all other questions that will show you are under age and there are too many to explain here. To answer the question of what they can do to you, nothing because you won't make it that far.

The truth would come out when you file your taxes as the age associated with your SSN wouldn't match the age of the person who owns the investments. I'm not sure of the specific penalties, but you could probably expect to have that investment income seized or be fined and asked to correct your personal details on the account.

would be difficult to lie about your age as you would have to give tax information such as your SIN number (at least that is what it is called in Canada...your Social Insurance Number)

then if you were to pull this off you would be trading illegally AND defrauding the US government of tax dollars....trust me...you don't want the IRS targeting you

Well they will require you personal information, and if you give someone else's info, you can go to jail for that.

The federal government could seize your investment gains.

So I'm going to open an account for online investing, so if I LIE about my age, how much trouble would I be in, and what would happen? God Bless; your answer is GREATLY appreciated.