The simplest answer is sleep. Over time in storage and especially when exposed to oxygen and heat, THC breaks down into CBN (Cannabinol). While not overly intoxicating, it is worthy of boasting over its ability to turn insomnia into a flimsy memory of what once was.
As research continues with more and more regions of the world legalizing cannabis, it might be uncovered that it isn’t CBN that sedates but the aging of terpenes that like THC alter with time. Wherever the sedation comes from, CBN or terpenes, old weed might not get you high but pairs beautifully with a pillow and blanket.
You might not want to purposefully store your cannabis for years on end, but if you do happen upon old weed don’t throw it out. Its use has simply changed into a good night’s sleep. Think of it as your friendly sleeping pill.