It depends on how you see the two parts connecting. They are two sentences that work together and they can be bridged in different ways:
It's sad (that) all these older guys (are) partying way past their prime.
What is sad is all these older guys (who are) partying way past their prime.
BTW, 'way' here means 'long' (time)
I'm not exactly sure what your question is looking for, but does this help?