Warner: Until recently, most competitive swimmers were high school or university students.
Warner's Evidence
Until recently, most competitive swimmers were high school or university students. Now, more and more competitive swimmers are continuing well beyond their university years.
Warner's Conclusion (Clearly)
Better training regimens have allowed today’s competitive swimmers to stay fitter longer than swimmers of the past.
Young's Conclusion
Not necessarily [disagreeing with Warner's conclusion].
Young's Evidence
No one has the time to both be a competitive swimmer and hold an outside job. But unlike in the past, today’s competitive swimmers can make a living at their sport.
Evaluate
The question stem tells us that Warner is making an argument, so we should look for evidence and a conclusion. Their first two statements are statements of facts, and "clearly" is a conclusion indicator. It might take a minute to get used to the fact that words like "clearly" are used as conclusion indicators on the LSAT, but what it indicates is that Warner believes this claim is clear and obvious, even if you, I, and Young see some potential flaws in Warner's reasoning.
Warner's argument offers an explanation for a curious fact. Why are more and more competitive swimmers continuing to compete well beyond their university years? Warner believes that it's due to better training. But there could certainly be other causes, and Warner has done nothing to rule out those alternatives.
Young's first statement, "not necessarily," indicates that they are disagreeing with Warner. But what exactly does Young disagree with? There are basically three options: Young might disagree with Warner's conclusion, Warners evidence, or the reasoning that Warner uses to connect the evidence to the conclusion. "Not necessarily" looks like a direct response to Warner's conclusion. And Young's other statements about swimmers making a living indicate that Young is suggesting an alternative explanation: swimmers can continue to compete after university not because of better training, but because the can make a living at the sport.
Goal
Look for an answer which states that Young offers an alternative explanation that contradicts Warner's explanation. Watch out for answers that describe situations which aren't actually happening in their conversation.