A good heuristic leads to a reduction in the overall cost of a decision (combining the cost of making the decision with the cost of the consequences if you get it wrong).

A heuristic like "it's risky to rent a car to a male under 25" saves a lot of cost in terms of making the decision (background checks, accurately assessing the potential renter's driving skills and attitude towards safety, etc.) and has minimal downside (you only lose a small fraction of potential customers) and so it's a good heuristic.

A heuristic like "a 26-year-old working a clerical job who makes novel statements about the fundamental nature of reality is probably wrong" does reduce the decision cost (you don't have to analyze their statements) but it has a huge downside if you're wrong (you miss out on important insights which allow a wide range of new technologies to be developed). So even though it's a generally accurate heuristic, the cost of false negatives means that it's not a good one.