It's no wonder most people go deaf as they get older...
This is a loudness reading from my car, sitting in the parking lot idling. Loudness was recorded with an iPod Touch 5th generation using Decibel 10th app by SkyPaw Co. Ltd (excellent free app). The stereo was playing a Supertramp song at low volume. Measurements were taken from ear position in driver's seat. The drop in volume at the end of the graph is the car idling with no radio. There is about a 45 decibel difference. Meaning the radio was the equivalent audible volume of a normal conversation according to gcaudio.com: http://www.gcaudio.com/resources/howtos/loudness.html
Also, the idling car, being at about 45-50 decibels, should serve as a good baseline reference. Being close to most of the sound types of the same decibel levels listed in the reference articles* should indicate a reasonable accuracy of metering.
Given that this is the case, and the fact that driving will introduce further base level noise, and also that most people listen to music louder than a normal conversation, it is my assumption that most people probably listen to the radio around 95-100 decibels. Keep in mind that decibels are not linear. In other words, 40 decibels is not twice as loud as 20 decibels. Instead, decibels are a logarithmic unit of measurement. Therefore, a change in power ratio by a factor of 10 is a 10 dB change (http://en.wikipedia.org/wiki/Decibel#Power_quantities).
At 95-100 decibels, you are left with about 1-4 hours of safe exposure per day based on slightly varying, multiple sources of recommended safe decibel levels*. Cumulated over 40-60 years, it is perhaps understandable that most people have hearing loss from listening to music in the car while driving.
So, next time you're rocking out to your favorite tune, perhaps limit your listening to a few songs and switch to something quieter? Or maybe try adjusting the volume to the lowest possible level that you can hear the music well and try to see if you can enjoy it, knowing your are saving some hearing! :-)
As for night clubs, concerts and other loud situations... Buy some earplugs! Seriously. Is all that partying worth being deaf? Who says you can't party hard with some discreet earplugs? There are plenty of options out there including models that retain overall sound quality, while reducing decibels levels evenly (unlike typical earplugs): http://www.etymotic.com/hp/erme.html
Protect those ears and enjoy all that great music... for life.