[colanderman]

Statistics don't work that way. Applying an aggregate metric (life expectancy) to an individual is what gets you statements like "Babe Ruth and I hit a combined 714 home runs", or beliefs like colonial Americans all dying by age 30 (they didn't; the average is heavily skewed due to child mortality).

Wearing a helmet doesn't increase the number of minutes of your life like you're winding a clock or something. It decreases the probability that you're dead by 40 from a preventable tragedy, leaving your parents without a child and your family without a parent.