It's divided into two, so it doesn't work perfectly as a grammatical unit. I'd put 'who are', but I'd change the full stop (period) into a semi-colon to join the two parts:
It's sad, isn't it; all these older guys who are partying way past their prime?
To be honest, Wendy, you're going round in circles with this sentence and I'm not sure that you're going to get anything from it.
