That is up to each individual person. Personally I hate it that someone decides how long my hair should be when I am at x age. If I am 90 years old and want it have it long enough to drag the floor, then that is my business and none of yours.
I am over 40 and have hair that is now mid-back. I did have it down to my tailbone, but cut it back because it really, really needed it. I do plan on growing it back out, but only to just below waist-length.