fantasy1989

What the argument says is that:
food scarcity --> reaching rereproductive maturity later than normal, therefore the average age to reach reproductive maturity will be older than normal.
excessive hunting --> reachineg reproductive maturity earlier than normal, therefore the average age to reach reproductive maturity will be younger than normal.

Apparently, the average age to reach reproductive maturity BEFORE the extinction was the NORMAL average.

fantasy1989

Read this sentence: since there are fossilized mastodon remains from both before and after mastodon populations declined,

So you have two populations: before and after the decline. From the population before the decline, you can obtain the NORMAL age to reach reproductive maturity. Then you can obtain the average age to reach reproductive maturity AFTER the decline. Compare these two number. If the former is bigger than the latter, excessive hunting is the cause. If the former is smaller than the latter, food scarcity is the cause.