Different eras have different minnows. Neither NZ nor India won a single test against anyone in his career. Hammonds average drops to 55 by your criteria, same as Kallis with the same exclusions.Start Date: 21st birthday (3 years after 18th birthday - picked too early for most players)
End Date: Day before 37th birthday (3 years before 40th birthday - terminal decline for most players)
Remove minnows (Bangladesh, Zimbabwe, Ireland, Afghanistan).
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Hammond is right up there for 2nd best batsman of all time. Basically Steve Smith before Steve Smith.
I can see why people might pick him over Kallis considering Hammond's bowling standard/average was the same as Kallis post 2004 when Kallis' batting really kicked into gear.
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Kallis peaked as a bowler in 99-02. He was averaging 62 as a bat in that phase.
I don't have a problem with saying Hammond has a case for being the second best bat since Hobbs. Kallis definitely doesn't. Era makes it tough to compare though. I wouldn't be 100% confident in calling Hammond better than some modern bats who were worse than Kallis. Hard to rate a bat who hasn't succeeded against quality pace, even if that is (mostly) no fault of his own.