Stretching - Now that's a better word. I never meant to imply that he was damaging his voice.
That sounds pretty logical actually - I experimented with that for a minute and you're right. You're a real singer anyway! Thanks for the heads up.

I think it has a lot of importance how you're singing - For instance, when you're at a gig watching a band you love perform you sing along and scream and stuff, yes? Afterwards your voice will be sour and you can't g overy high without it cracking. You've also lost your falsetto register - Falsetto is the first to go with age as well as straining - although with a few days rest you're pretty much back to normal aside from coughing a bit when reaching a fairly high note.
This is how you damage your voice and what damages most easily is the upper register which will also be lowered with age. Your upper register is fragile and your body's ability to recouperate will lessen with age - Instead of reaching all notes no problem at age 25 you might only be able to do that every third night at 57 and every now and then your body might behave like you're 20 again and you'll sound incredible (Halford being an example here, he can range nowadays from sounding good live to horrible to fantastic - It just depends on his day form. He rarely sounds fantastic anymore though but there are a few occassions).
So. Let's pretend I can sing and let's pretend I'm famous. I can either do it like Halford, try for these incredible screams etc which are quite hard (talking pre 2002 Halford) or I can sing easier stuff which I can sing no problem when I'm old or having a bad day. If I do the second option it will go easier on my voice and I'm in less risk of harming it and I will sound pretty much the same when I am 60. Singing like Bruce isn't easy (I wouldn't know that by experience though!) and singing the way he does when you have a cold etc might harm your voice (actually, singing anything where you have to stretch a bit when sick might harm your voice - It's easier to hurt your voice when you're sick and trying to sing like when you're not). I think what the instructor might have meant it's easy to slip when you're not in perfect shape and harm your voice when singing like Bruce 500 show in a couple of years - I mean even when you're 25 you will have a few bad nights and you might end up hurting your voice if you're trying to sing full ability when you have a cold or something.
However, now that we have a real singer here, Stan, he might better answer your question.