well, as far as the age/hair thing goes- if you have hair that looks good long, then by all means keep it as long as you like- but if you just like long hair and your hair is too fine and thin to look good that way, then you should cut it. The weight from longer locks will only pull your hair down and make it look even thinner and wispier. Going shorter will give it more body.
My hair is fine but I have alot of it- it always looks better with a bit of build up and I use a few styling products to make it seem more textured and thicker- mainly Garnier Fructis Surf Hair, both the spray on texturizer and the paste, both of which I use when my hair is still wet. I also use the big velcor rolllers when hair is almost dry for body, let them sit in my hair till it dries, then hit it with a spray on wax, the new one from John Frieda sheer blonde is awesome! The Surf Hair is the real secret weapon, though. A few other tips- let your hair be almost dry before applying the products, then even drier before putting in the rollers or blowdrying. Also, a shorter cut with lots of cute, choppy layers ismuch more flattering for fine hair than trying to make it look attractive long.