With age comes a decline in, well, just about everything - energy levels, mental capacities, libido, you name it. If it's good, or good for you, most likely it declines with age.
--- I'm not sure what the author means by saying "if it's good, or good for you"?
Top answer
"it" refers to any attribute or quality. If it's good, most likely it declines with age. (normal meaning of "good") ...
— GPY
"it" refers to any attribute or quality.
If it's good, most likely it declines with age.
(normal meaning of "good") ...
and ...
If it's good for you, most likely it declines with age.
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It is not the decrease or decline that is beneficial, but the quality or attribute itself.
"If it [= any attribute] is good, or good for you, most likely it declines" -- it is a way of saying "Anything that's good, or good for you, most likely declines."