The answer is quite simple, because it lacks proper semantic sequential order. Semantics is the part of linguistics that focuses on meaning. A paragraph is usually chunks of meaning that need to be properly organized in order for their different meaning units to convey the themes and the message appropriately.
If this paragraph starts out by stating that by giving community service, teenagers also benefit themselves; semantic logic dictates that the next lines will immediately provide arguments/evidence of such assertion. However, the order is incorrect because it does not follow with that since it immediately focuses on the benefits for the elderly, when according to the first sentence; the intent of the paragraph is to appeal to the usual and prevalent sense of self-gratification of adolescent readers and the elders come second.
