In Romeo and Juliet, what is the device/importance of "now old desire doth in his deathbed lie, and young affection gapes..."

The device is called personification.  Personification is when one attributes human actions, ideas or emotions to non-human objects.  In this line, "Desire" is depicted as old and lying down in a bed awaiting death.  Now, a person can do such a thing, but an event, like aging and death, cannot.  The same is true for "young affection,"  which is portrayed as being eager to have the old guy, desire, be bumped off so youth can take what it considers to be its rightful property, "gaping" while s/he waits. 

These choral lines allude to the youthful desire of the lover's who will not be swayed by the "old people," ie, their parents (and anyone else who stands in their way.)  Bah!  What do those old people know?  In the words of Tracey Ullman, "They don't know about us / And they've never heard of love."