i usually like anything between 16 and 22 inches. And no, i don't think your age matters at all. However, i have a long neck and was told that most length look good on me
18" and I have a LONG neck. I've read people should wear long necklaces to lengthen a short neck and short necklaces to break up a long neck. When I was younger I wanted ALL my necklaces to be 15-16 inches, but at about 33 I started prefering slightly longer necklaces. I like 18-22" and a few longer ones occasionally. The exception is my bezel set diamond solitaire. It's 16" and looks nice at that length and doesn't interfere as I layer it will all my 18" necklaces.
I usually wear a 16" because I like the way the neckline looks. I have a long neck, I guess, so I suppose it breaks it up...although I never really thought about it that way!
I am petite and usually prefer a 15" (16" is too long). However, I've started playing with sautoirs and now I'm all over the place. I totally blame the super fun Hermes Farandole necklace for this development.