> A study asked different age groups to mentally count 120 seconds. People under 30 averaged 115 seconds; those over 50 just 87.

This can be interpreted in two ways, which lead to diametrically opposite conclusions.

1. I say "start" and you say "stop" when you think 120 seconds have passed, and the result (115 or 87) is the actual elapsed time.

2. I say "start" and some time later (which happens to be 120 seconds, but you don't know that) I say "stop" and ask you how much time you think has passed. The result is your response.

On the first interpretations, everyone's internal clock runs fast (everyone gets to 120 ahead of the actual time), and an old person's run faster than a young person's. On the second interpretation, everyone's mental clock runs slow and an old person's run slower than a young person's.