The second to last stage of man in the poem "The Seven Ages" is all about man slipping into decline gradually, and heading towards a more "childlike" state once again.
The wearing of "slippered pantaloons" is a reference to jesters or fools who would wear such attire... Signalling that man is more easily fooled at this age.
He also wears spectacles because he struggles to see, and his big manly voice is turning to a more childlike and whistling tone.
