18-year-old pop singer, Justin Bieber, became famous at age 15, for his high pitched voice, glossy toss worthy hair and big smile. As Bieber aged, he, like his voice, went through puberty, which deepened his vocal tone. As a result, Bieber sometimes strains to hit high notes from his older songs, recorded when he was 14-years-old.
Singers should beware of straining when they sing. It will damage the voice over time and cause bleeding. Producers should beware of giving artists material to sing that is out of their vocal range or transpose notes for younger vocalists going through puberty, whose voices are changing. It's not that he cannot sing, but the notes need to change to accommodate his voice.