Define "old". I've been told that long hair doesn't look good on women older than 40 due to the fact that it gives the impression that they look older than their age. But then again I might be completely wrong.
Some of the most beautiful women who have inspired me are 50+ and have waist length hair. Age is but a number when it comes to hair. Ultimately, you should keep a hair length that it easy to maintain and that actually suits you.