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Who should pay taxes in my typical case?
The bottom line is you broke the law.  While this is pretty much victimless, it is none the less a violation of the law and should be avoided in the future.  I would have not agreed to this as a parent and it sets a bad precedent.  As such I would avoid trading and move the money into cash until you turn 18.   Once you turn 18 you should transfer the money into an account of your own.  From there you may proceed as you wish. As far as paying taxes, of course you need to pay them.  Your mother did this as a favor to you and by doing such you caused her tax bill to rise.  As a gesture of goodwill you should at least provide her with half of the profits, not the 15% you propose.  Fifteen percent would be the "I am an ungrateful son" minimum, and I would seriously consider giving all of the profits to her.