Should I take a student loan now, if I intend to become a mathematician? I intend to pursue my degree in Mathematics and Physics at Ecole Polytechnique in France. The institution has offered me an interest free loan of 18,000 euros, which has to be repaid three years after I complete my bachelor's degree. 

Since I am passionate about basic sciences, I hope to join a five year PhD programme after my degree and this usually comes with a decent scholarship/stipend. Approximate salary for PhD scholars in Europe is about 1300 euros, though this might vary from institution. Right now, I am not sure whether this salary and the one received during Post-Doc might be enough for me to repay these loans and therefore, I wish seek advice on this matter from the Academia Stack Exchange community which, I believe, will be in a better position to answer  this question since most members are pursuing a career in Academia.
Before answering whether you should take the loan you need to ask yourself some basic questions.


Would I go to the school if there was no loan available?
How would I pay for the school without the loan?
Are you good at budgeting your money?


If you would not go to the school without the loan, the answer to the big question depends on why the loan influence's your decision. The answer is almost always that debt is bad. If you would go to the school without the loan, have a means of paying for school and are good at budgeting, then take the loan and put it in a secure investment. At the end of the 3 years, cash out the investment, pay off the loan and pocket the interest. If you are bad at budgeting, you want to be careful about taking on debt and the answer is almost always debt is bad.