I’m looking to create a playlist of songs about mortality – not necessarily about death (although death could certainly be involved), but more about the fleeting nature of life, how it’s all gone in the blink of an eye, about how time waits for no man, etc., etc. You know, cheery stuff like that. 
For instance, a good example (and in fact the song that got me thinking about this) is 100 Years by Five for Fighting.
Any others? All genres of music are welcome.
Had a bunch more in mind but forgot them by the time I tracked down that link. Maybe back later…
Well, there’s “Three Bells” by the Browns, if you’re willing to include real schlock.
I’ve recently started listening to Bone: Thugs-N-Harmony again and the following song is what got me into them into the first place and it kind of fits what you’re asking about.
Tha Crossroads
Dust in the Windby Kansas
Afterimageby Rush
Dreamlineby Rush
Time Stand Stillby Rush
(Yeah, I like Rush. And yeah, they write a lot of songs about that topic) 
Well, I’m done contributing to the thread. You’ve submitted the others I was wanting to add (“Afterimage” and “Time Stand Still”). TSS is one of my all-time favorites.
Practically anything by Oingo Boingo!
We Close Our Eyes
No One Lives Forever
Dead Man’s Party (?)
…and many others.
I Will Follow You Into the Dark - Death Cab For Cutie
The Luckiest - Ben Folds
They’re both kinda love songs with a hey-you’re-mortal twist…
Kurt Weill’s “One Life to Live,” from Lady in the Dark: One Life to Live - YouTube
The Ballad of Hollis Brown - Bob Dylan
Alone Again (Naturally) - Gilbert O’Sullivan
I’ve Been to Town - Frank Sinatra (I prefer the version by Glenn Yarbrough)
It Was a Very Good Year - Sinatra
Don’t Fear the Reaper?
“Older” - They Might Be Giants (lyrics, song)
“The Lament Of Desmond R.G. Underwood-Fredrick IV” - Steve Taylor (lyrics, song)
arguably, “All Things Must Pass” - George Harrison
And my boy Dan W. Quinn, singing the catchy “More Work for the Undertaker!” More Work for the Undertaker--Dan Quinn 1902 - YouTube
Who Wants to Live Forever - Queen
Time - Alan Parsons Project
Time - Pink Floyd
The Fire Inside–Bob Seger
What? Nobody’s mentioned ‘Seasons In The Sun’ yet?
Lay Me Low–Nick Cave
1952 Vincent Black Lightning
Old 97s - You Were Born To Be In Battle
Deteriorata - National Lampoon
Ha Ha You’re Dead - Green Day
My Way - Frank Sinatra or Sid Vicious