As you know in a Public Limited Company (PLC) anyone can buy shares in them. These shares are sold on the stock market and because they can go up as well as down, this is deemed a form of gambling, hence why you have to be 18 to do this with real money. 

However if you quite fancy having a try and seeing how much money you could have made or lost if you had invested, then my simple spreadsheet should help you do this nice and easily. You can download your own copy of this and make ammend that copy.