The calculator suggests $5,021 for food, but for me I’d only shop at high-end grocery like Whole Foods and buy organics whenever possible. That’s clearly not enough. On the other hand it suggests $1,792 for internet and mobile which is about double what I actually pay and I have both unlimited mobile data and unlimited home data. Then it claims medical costs of $2,890. For a fit individual with good employer-provided health insurance, that figure should be almost zero.
Ultimately the amount one spends for living depends very much on one’s preferences and these calculators are approximates. I believe you when you say many young people can live for much less, but that doesn’t invalidate the calculator.