How can I tell if a circuit breaker is worn out?
Breakers are supposed to be good for a fairly large number of triggers, and they are supposed to fail open when they fail. Is there a chance that you were running at a heavier overall load when the second incident occurred? I'm thinking that if you were say within 10 amps of max on the main breaker just due to normal load (air conditioners, perhaps?), and caused a short on the secondary (a 15 amp circuit, perhaps), you might have triggered the main to trip first by simply overloading it, before the secondary could draw enough to trip.

If you really think the secondary might be bad, then by all means replace it, but no, you don't have to replace the main just because it tripped. As long as it feels normal when you move it back into position (flip some other breakers to see how they feel - there shouldn't be any slop in the movement), you'll be fine. 

The last several times I've seen breakers fail they've all failed open.