100m: 30 year old. Probably something like mid 14s for the 30-year old vs. low 15s for the 13 year old. I've seen lots of fat guys who could sprint fast.
Mile: tossup. The man and the boy will be around 9 flat.
5K: 13 year old. The borderline obese man will struggle to finish the race.
Discus(s)

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