Hi @Romain Amrani, and welcome.
I wish the best of luck to you - thinking about investing at an early age is a great way to set yourself up for financial success in your future!
Real Estate can be a great career and finding a mentor to help you along your journey sounds like a good place to get started. Your young age isn't really much of an issue - as soon as you turn 18 you're a legal adult and banks will view you just as anyone else.
As far as getting a traditional mortgage loan though, you'll still need the same 2 factors everyone else does: credit and income. Since you're a student you likely will have $0 income. And since you've just turned 18 you have no credit history.
My advice at this stage if you foresee a career in RE investing is this: begin establishing a credit history as soon as you turn 18. Get a credit card, charge a bit each month, and then pay it off in full every month. It will not take you long to establish good credit if you do that.
If you intend to use a traditional mortgage to fund your deals, you're right that you'll likely need your mother's help to qualify since you do not have income to show. The legality isn't an issue; don't worry about that. If she is willing to be a co-signer or joint borrower on your loan, then that will get you approved AND will help you build credit history.
HOWEVER - this might all be a moot point if you're trying to flip homes. Most investors need to resort to either all cash or hard money / private loans to fund flipping deals. This is because most properties that are 'flip-able' are in bad shape, and will not satisfy a traditional bank's condition requirements for use as collateral.
My last (and most important!) bit of advice is this: enthusiasm in a career is great - in fact, to be a successful investor it is critical. But no career is ever worth being the sole focus of life. Never let yourself become so worried or obsessed with a job that it keeps you up at night :)