Black marks are not a sign of aging.  It could be that you banged it or had a splinter and it left a bruise under the nail.  That's the usual cause of black marks.  I wouldn't worry too much about anything still being in there because it would be painful.  Of course, if it gets bigger or doesn't seem to grow out, then you should probably mention it to your doctor.

This could be way out in left field, but I had some nail discoloration a while back and come to find out it was because our household water had a ridiculously high iron level. We got the level under control and my nails went back to normal.

Are black marks on your fingernails just another sign of ageing or could this be some other condition and, if so, any idea what? I have what looks like a splinter in the middle of one nail.