In a nutshell, I think that this is actually an uplifting song, and proves that Thom is happy he still has his youth, and that when (if) he gets old and senile, to use this song to remind him of how he used to be happy, cool, and loving during his youth.  

Here is the verse by verse interpretation...

"if i get old i will not give in 
but if i do 
remind me of this ...
remind me that 
once i was free 
once i was cool 
once i was me "

In this verse Thom shows his desire to stay young and "sane" and that if he ever starts "to lose his marbles" he will fight it, but when he inevitably loses his youth, to remind him of the days when he was his true "cool" self. and when is brain wasn't obscured by equivocalness.

and if i sat down 
and crossed my arms 
hold me into 
this song 
knock me out 
smash out my brains 
if i take a chair 
start to talk shit 

This verse is pretty simple to interpret, stating that if he is too lazy to be who he used to be, and just wants to sit down and is too stubborn and recalcitrant to do anything, make him live up to this song, and not let him become this way.  And if after that, he is still obstinate and intractable, then he shouldn't live at all, so he should be killed.

if i get old 
remind me of this 
that night we kissed 
and i really meant it 
whatever happens 
if we're still speaking 
pick up the phone 
play me this song

This is further reinforcing the motif of the song with a caring and romantic overtone in this verse.  It says when he gets old, don't let him forget of the days when he really cared about you, and that he "really meant it" when he kissed you. (whoever this may be)  The song then closes by saying, no matter what, if we are still in touch, pick up the phone and remind of this by playing him this song.