It is true that in old men there is a large percentage of slow growing mostly harmless prostate cancers, but you can also get it if you are younger (or it can be aggressive) and then treatment is possible (and required). The advantage of doing a PSA test is however not clear cut, because having a (slightly) elevated PSA can be caused by a benign enlarged prostate or prostatitis. This is why this new test is worthwhile (if it really works).

Source: I'm 47 and have prostate cancer (metastasized, so incurable but there are still a number of treatment options to improve quality and quantity of life).