Long, loose hair is a young person’s style, unless it is long enough to be put up (a very old-fashioned look, in general). Long, full, shiny, richly hued hair is one thing, and long, fine, thin, wispy gray or white hair is another. Some older women can wear long hair well, but the odds are against it just because of the physical differences.
Also there is the fact that long hair can be a very effective attractor to men, so much so that the Bible and many cultures have something to say about it (for example, that married women must cover and/or cut off their hair). It can be presumed that matrons have a reduced interest in announcing their candidacy for men’s attention.
Speaking personally, I cut my hair short for the first time ever only about 15 years ago. I thought it looked great, and it was widely complimented. But it did take a lot of fussing—much more, in fact, than when long. That is one reason why now as I am preparing to leave the work force I am going to let it grow out again. As for coolness, unless your hair is tiny short, putting it up in hot weather is cooler than any style that leaves damp, stringy wisps clinging to your neck.