I've been recruited to put together a compilation CD for a friend's 50th bday. The theme is kind of loose, but should relate to age/getting older or perhaps staying young

Off the top of my head:
18 - Alice Cooper
100 Years - Five For Fighting
Cats In The Cradle - Harry Chapin

Looking for any and all ideas, though metal and/or rap probably wouldn't be the best fit...

It Was a Very Good Year- Frank Sinatra

Mellencamp- Cherry Bomb, Your Life is Now

Here are some of my current favs.

Numb Encore- Linkin Park/ Jay Z
Apoligize- One Republic
Buy U a drank- T-Pain/Young Joc
Ghetto Gospel- 2pac/ Elton John
Arab Money- Busta Rhymes

Got just the song for you. It's entitled "Middle Age" and is the opening track on Steve Forbert's Strange Names & New Sensations.

Old - Paul Simon

jerry lee lewis- middle age crazy
this is the perfect song
http://www.youtube.com/watch?v=sj0o0QO2riY

Tom Cochrane - "Boy Inside The Man"

Not to be confused with the Michael Jackson cover version.

Michael Jackson did a cover of that? (Or are you just joking?) I know that the New Monkees did - and surprisingly, it's just as good, if not better than, Cochrane's original.

Only The Good Die Young - Billy Joel
Old Man - Neil Young

When I'm 64 - The Beatles

Pink Floyd's "Time" maybe?

"Why Don't We Get Drunk and Screw" by Jimmy Buffet. OK, doesn't really have anything to do with growing older, but when your friend makes a comment about it, just wink and say "thinkin' of you".

There's always "Aqualung" by Jethro Tull or "Don't Stand So Close to Me" by the Police, about older men.

Or, back to Jimmy Buffet, "Growing Older but not Up" with the chorus:

I'm growing older but not up
My metabolic rate is pleasantly stuck
So let the winds of change blow over my head
I'd rather die while I'm living then live while I'm dead

Edit: Here's a list of 25 songs on aging. From this list, Steely Dan's "Hey 19" would work well... about an older guy who meets a younger woman:

Hey nineteen
No we got nothing in common
No we cant talk at all
Please take me along
When you slide on down

http://www.slate.com/id/2199897/pagenum/all/#page_start

Toby Keith - As Good As I Once Was

No, that was a joke.

Excellent choice!!! Add to that Floyd's "Free Four" found on Obscured By Clouds album.

Free Four lyrics

The memories of a man in his old age
Are the deeds of a man in his prime.
You suffle in gloom in the sickroom
And talk to yourself till you die.
Life is a short, warm moment
And death is a long cold rest.
You get your chance to try
In the twinkling of an eye:
Eighty years, with luck, or even less.
So all aboard for the American tour,
And maybe you'll make it to the top.
And mind how you go.
I can tell you, because I know.
You may find it hard to get off.
You are the angel of death
And I am the dead man's son.
And he died like a mole in a fox hole.
And everyone is still in the run.
And who is the master of fox hounds?
And who says the hunt has begun?
And who calls the tune in the courtroom?
And who beats the funeral drum?
The memories of a man in his old age
Are the deeds of a man in his prime.
You suffle in gloom in the sickroom
And talk to yourself till you die.

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Only The Good Die Young is about trying to talk a virginal Catholic school girl to give it up. It has NOTHING to do with aging.

Old Man, on the other hand, Nice call there.

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"Reminiscing" - Little River Band (with lyrics)

Jim Croce - "Time in a Bottle"
Bryan Adams "Summer of '69"

I second this, along with:
They Might Be Giants - I Hope That I Get Old Before I Die

Sometimes I feel like being wispy
And once in a while I feel like being dry
But we're doomed and we're drowned by this feeling we surround
So I hope that I get old before I die

Ohhhhh
It's a long, long rope they use to hang you soon I hope
And I wonder why this hasn't happened
Why why why
And I think about the dirt that I'll be wearing for a shirt
And I hope that I get old before I die

Clear off the kitchen table darling
For on the kitchen table I must lie
I'm just tired for my wife just served the banquet of my life
And I hope that I get old before I die

Ohhhhh
It's a long, long rope they use to hang you soon I hope
And I wonder why this hasn't happened
Why why why
And I think about the dirt that I'll be wearing for a shirt
And I hope that I get old before I die

Ohhhhh
It's a long, long rope they use to hang you soon I hope
And I wonder why this hasn't happened
Why why why
And I think about the dirt that I'll be wearing for a shirt
And I hope that I get old before I die

Yeah, it's actually "Man Inside the Boy."

How about "End of the Line" by the Traveling Wilburys? I think I suggested that a while back when someone else asked for such songs.