Though foolishly he lost the same (2)
Poor Adam was so dumb that he bungled away every good thing going for him: all the "wealth" and "store" created just for him. The verb "lost"—pretty mild considering the scope of what happened—makes Adam seem like a child, some poor second-grader who lost his sandwich on the way to school.
Decaying more and more,Till he becameMost poor (3-5)
Herbert makes Adam sound like a corpse, "decaying" slowly in sin and pain and poverty, until everything is gone. The repetition of "more" is like a verbal nudge in the ribs: Herbert wants us to know that this guy's lost it all.
My tender age in sorrow did begin (11)
"Tender" here doesn't mean lovey-dovey; it means young and fragile. And that young fragility isn't helped any by confronting a world of sorrow. The speaker underscores his vulnerability.