Complex sentence: The oldest of Ryoki's acquaintances at 24 years old, though he looks much younger, ten years before the beginning of the story he was part of a fearsome gang of juveniles known as Platinum, enjoying a lifestyle of savage brawling.
Split into simpler sentences: The oldest of Ryoki's acquaintances at 24 years old (though he looks much younger). Ten years before the beginning of the story, he was part of a fearsome gang of juveniles known as Platinum, enjoying a lifestyle of savage brawling.