The only argument I can see for its "usefulness" as a statistic is if you want to imply that the old are less useful than the young. It's not about "usefulness

The only argument I can see for its "usefulness" as a statistic is if you want to imply that the old are less useful than the young. It's not about "usefulness", it's that saving the life of a young person produces a larger benefit in terms of expected future years of life. Take the extreme case: if a building is on fire and you can only rescue one person, would you be indifferent between saving the life of a 10 year old child versus a 90 year old with terminal cancer?