As almost everybody, but a teenager knows, life can be very unpredictable and unfair. Which is why when you choose a financial planner or retirement calculator, you should be presented with probabilities, not absolutes.
For instance, if you save $100 per week for the next 35 years starting with nothing, then you’ll have an X% chance of reaching your retirement goals. The greatest factor in this is the investment return and NOBODY is able to predict this. They can make a guestimate as to possible scenarios, but that’s it. Here is a link to just such a retirement calculator. Play with the variables to see how dramatically things can change. Your final assessment should be conservative though.