current age != lifespan https://en.wikipedia.org/wiki/Life_expectancy https://en.wikipedia.org/wiki/Life_expectancy > if tomorrow we decided to build a new br

current age != lifespan https://en.wikipedia.org/wiki/Life_expectancy https://en.wikipedia.org/wiki/Life_expectancy > if tomorrow we decided to build a new bridge next to every old bridge, average lifespan would half. The only way building a bunch of new bridges changes lifespan is if we changed how they where built. Figuring out lifespan rather than age is at best an approximation after looking at the bridges that didn't survive and the condition of bridges that do, but the uncertainty is low.