Many years ago, before everyone had the option of carrying a dB meter in their smartphone, I had a friend who liked to play his music "Loud." And what constituted loud for him? It was so far back in the mists of audiophile time that I never measured his peaks with a meter, but I do know that he was constantly melting down amps. A "normal" listening session often lasted under 15 minutes since by that time many of the amps he used went into thermal overload and shut down. I also remember seeing his pants legs move from the SPLs...
Another time, that I'm still wishing I had not experienced, was the MC-5 band, live. It was my first exposure to Marshall guitar amps turned up to 10 (no, there was no 11) and even with wax ear-plugs (that was all we had in those days) my ears rang for a week. I blame Wayne Kramer, their lead guitarist, personally, for a good part of my upper frequency hearing loss. I'm only good to 13KHz, now.
Early in my career as a photographer I began wearing earplugs at concerts. Along with film, earplugs were an essential part of my on-location kit. But, even with plugs, eventually all those loud sounds over the years can take their toll. But I got away relatively scot-free compared to the hearing loss horror stories I've heard about people who served in the armed forces and were exposed to live-fire situations.
But what do you consider loud?
Nowadays there is no reason that we can't protect our ears with more precision. First off, earplugs are readily available in a wide variety of types. Yes, there's still the waxy ones you can mold, but there are also plugs with built-in electronics that will attenuate loud sounds while letting soft ones through. And with those ubiquitous SPL meter apps for smartphones, there's no excuse for not knowing exactly how loud a sound is, and whether its reached the point where plugs should be employed.
I will admit that I don't listen to music played as loudly as I did in my youth. Rarely do I turn the volume up above 95 dB A-weighted peaks. Most of my listening is lower, and in my current room-based systems I usually find the levels peaking in the low 90's. I wish measuring the levels coming from headphones was as easy. I fear that like many audiophiles, I probably do listen to cans at higher levels than I should for long-term ear health. I've read alarmist articles that claim a whole generation of listener's have been irrevocably damaged from earphone listening. I hope not.
Of course, if you try to listen at too low a volume level the old Fletcher-Munson low frequency perceptibility curve can come into play. As music gets quieter it becomes harder for us humans to hear low frequencies. That's why many vintage receivers and preamplifiers have a "Contour" or "bass enhancement" controls. Unfortunately, most of these controls are a "one setting fits all" type which ends up being "one setting fits none." One of the nice advantages of a system that has digital signal processing is that you can, if needed, design and execute your own "loudness" settings for low-level listening.
So, in theory, no one should be subjected to excessive volume levels if they carry ear protection and use a SPL meter to measure levels. But some situations can't be avoided, such as when the fire alarms went off at an audio show a couple of years back. My fingers went into my ears immediately. But then I was faced with a dilemma - do I take my fingers out of my ears, exposing them to full-level nasty alarm, to get and install my earplugs, or do I just stand here, fingers in ears, like a statue, until it stops...my solution was to walk briskly to the stairwell, which had no alarm in the immediate vicinity and then install my plugs.
Yes, sometimes loud, nasty sounds are unavoidable, and just as no one gets out of this world alive, very few of us will go through life with our hearing intact. Protect what you've got, because there are no miracle cures for hearing loss - once it's gone, it's gone...