Basically it all comes down to transparency, which I guess makes lossless a bit relative. Which in effect comes down to inherent hearing ability vs equipment quality. Like SomeGuyDude suggested, if you have crappy equipment, you wont be able to discern a difference regardless of how good your ears are (age vs natural ability to hear). You will just have shit quality no matter what quality your source file is. But with more high end equipment, the ability of the equipment to distinguish differences in quality is improved, and therefore you are more likely to notice a difference, even given slight biological differences.
MOST of the time I am able to tell the difference between a 320kbps mp3 and one of a lower bitrate, but it really depends on the complexity/fullness of the music and the production. Therefore I always rip in at least 320kbps mp3 just to be safe. But the difference between 320kbps mp3 and lossless is usually just a subtle difference in clarity. Cymbals may ring with more brightness and sound more natural, instrument separation may be a little better, etc. Though unless the music is very textures/layered, I find it hard or impossible to tell any difference, even with the most hi-fi of my equipment (which isnt all that high-end tbh). So for the most part a 320kbps mp3 is completely satisfactory to my ears and equipment, and even if I can tell the difference, it is usually so subtle as to be insignificant.
I still continue to rip my favorites in lossless FLAC format though, even if it is just placebo more than half the time. I have over a terabyte of free hard drive space so for every lossless file I have a duplicate 320kbps mp3. Imo it is fully worth the effort of trying to enjoy the music I like in the best way I possibly can (without going bankrupt). I admit to doing a little bit of downloading, so some of my collection is slightly bitrate starved. Though as long is it isnt 128kbps or lower it is more or less listenable.
