1. So adverb [still] can be placed before participle phrase??
Still is usually placed before the verb (in this case partying), except for the verb to be, even if it used as an auxiliary verb. If the verb to be is being used as an auxiliary verb, it will be caught between the auxiliary and the main verb.
I am still here.
I still have the book.
We still haven't gone.
And yes, it can be followed by a participe phrase and many other types of phrases as well.
2.Way in older guys, partying way past their prime mean what, and modify what in the sentence??
Way, in this sentence means "very much" and it is modifying the word "past". therefore. the men are very much past their prime, they are too old for this type of behavior.