Again, apologies for pulling out an oldie, but an obvious choice would be "The Times They are A Changin'" by Bob Dylan. To some extent it is about changes in society, but it is also links people's happiness to how successfully they adapt to them. The music may sound dated to young twenty-first century listeners (I know from experience, having tried to use it in a US history class) but the lyrics could hardly be more radical:

Come gather 'round peopleWherever you roamAnd admit that the watersAround you have grownAnd accept it that soonYou'll be drenched to the boneIf your time to youIs worth savin'Then you better start swimmin'Or you'll sink like a stoneFor the times they are a-changin'.Come writers and criticsWho prophesize with your penAnd keep your eyes wideThe chance won't come againAnd don't speak too soonFor the wheel's still in spinAnd there's no tellin' whoThat it's namin'For the loser nowWill be later to winFor the times they are a-changin'.Come senators, congressmenPlease heed the callDon't stand in the doorwayDon't block up the hallFor he that gets hurtWill be he who has stalledThere's a battle outsideAnd it is ragin'It'll soon shake your windowsAnd rattle your wallsFor the times they are a-changin'.Come mothers and fathersThroughout the landAnd don't criticizeWhat you can't understandYour sons and your daughtersAre beyond your commandYour old road isRapidly agin'Please get out of the new oneIf you can't lend your handFor the times they are a-changin'.The line it is drawnThe curse it is castThe slow one nowWill later be fastAs the present nowWill later be pastThe order isRapidly fadin'And the first one nowWill later be lastFor the times they are a-changin'.

Marge Piercy's poem "Barbie Doll" comments upon the fixation of being thin and the pressures upon females to be pretty.  Its shocking message is a reminder of the frivolous, but often devastating, importance placed upon conforming to accepted views of beauty.

This girlchild was born as usualand presented dolls that did pee-peeand miniature GE stoves and ironsand wee lipsticks the color of cherry candy.Then in the magic of puberty, a classmate said:You have a great big nose and fat legs. She was healthy, tested intelligent,possessed strong arms and back,abundant sexual drive and manual dexterity.She went to and fro apologizing.Everyone saw a fat nose on thick legs. She was advised to play coy,exhorted to come on hearty,exercise, diet, smile and wheedle.Her good nature wore outlike a fan belt.So she cut off her nose and her legsand offered them up. In the casket displayed on satin she laywith the undertaker's cosmetics painted on,a turned-up putty nose,dressed in a pink and white nightie.Doesn't she look pretty? everyone said.Consummation at last.To every woman a happy ending.

For a poem, how about AE Housman's "When I was One and Twenty?"  That's about getting wiser in the course of a year or so.  For songs, from the past there's Bruce Springsteen's "Glory Days" and John Cougar's "Jack and Diane."  For the present there's Taylor Swift's "Fifteen" and "Never Grow Up."

I thought of a song called "This One's for the Girls" by Martina McBride.  Although it refers to girls and the changes in girls' lives, many of them can be applied to men too.

Here's a link to the lyrics:

http://www.azlyrics.com/lyrics/martinamcbride/thisonesforthegirls.html

The song describes a girl's life at 13, 25 and 42 years old and what changes.  At 13, the perils of high school are described.  At 25, the difficulty of getting a life started and hopes for the future.  At 42, grief for lost youth but recognition for accomplishmet.

The first poem that comes to my mind is "The Love Song of J. Alfred Prufrock", T.S. Eliot's famous work. This is a poem about looking forward to the changes that life brings, wondering about them, and taking a somewhat pessimistic view of what the aging process does to one's sense of possibility. 

As a parent I am often haunted by Cats in The Cradle by Harry Chapin.  It tells the story of a dad who's son idolizes him and wants to spend time with the dad.  As the song continues, the dad is constantly too busy to be a part of the child's life.  Once the child grows up and has his own life, the dad realizes the mistakes he has made when his son never has time for the father. 

http://www.lyricsdepot.com/harry-chapin/cats-in-the-cradle.html

I hope you don't mind an oldie but goldie, but David Bowie's "Changes" immediately came to mind. Although it's primarily about Bowie's own maturation in the music world, the lyrics can still be interpreted in many different ways.

I still don't know what I was waiting forAnd my time was running wildA million dead-end streetsEvery time I thought I'd got it madeIt seemed the taste was not so sweetSo I turned myself to face meBut I've never caught a glimpseOf how the others must see the fakerI'm much too fast to take that testCh-ch-ch-ch-Changes(Turn and face the strain)Ch-ch-ChangesDon't want to be a richer manCh-ch-ch-ch-Changes(Turn and face the strain)Ch-ch-ChangesJust gonna have to be a different manTime may change meBut I can't trace timeI watch the ripples change their sizeBut never leave the streamOf warm impermanence andSo the days float through my eyesBut still the days seem the sameAnd these children that you spit onAs they try to change their worldsAre immune to your consultationsThey're quite aware of what they're going throughCh-ch-ch-ch-Changes(Turn and face the strain)Ch-ch-ChangesDon't tell t hem to grow up and out of itCh-ch-ch-ch-Changes(Turn and face the strain)Ch-ch-ChangesWhere's your shameYou've left us up to our necks in itTime may change meBut you can't trace timeStrange fascination, fascinating meChanges are taking the pace I'm going throughCh-ch-ch-ch-Changes(Turn and face the strain)Ch-ch-ChangesOh, look out you rock 'n rollersCh-ch-ch-ch-Changes(Turn and face the strain)Ch-ch-ChangesPretty soon you're gonna get a little olderTime may change meBut I can't trace timeI said that time may change meBut I can't trace time

http://www.elyrics.net/read/d/david-bowie-lyrics/changes-lyrics.html