Tesia,
This is a hard one to interpret. The developing mind of a six year old sees the world differently than an older person and thus their dreams are less likely to reflect objective emotions from the waking life {dreams are reflections of our emotions, past and present}.

Being in a dark room is a common fear for a child. The lines were coming after you so that may fit with a fear of 'some thing' that you were afraid of. Thin and fat are opposites and in your dream in both cases you end up dying {dying in a dream usually means an end to some aspect in the dreamer's life}. The line could represent boundaries in the waking life. There may have been an experience early in life were certain boundaries were crossed {by someone else or yourself}. The dying could be a feeling of loss due to the experience. We must consider the fact the fear ended at the age of 14. It may be the childhood fear was removed because of maturity. Or something related to the experience no longer existed in your life at 14. Or it could be your pushed the experienced into the deep unconscious {dark room} so it would no longer bother your waking conscious mind. So many possibilities to mention. The least being a child of six can have dreams that are as much meaningless than meaningful {as is the case when you get older}.

Test the possibilities. What changed in your life when you turned 14 if anything? Were there any 'dark' experiences that could have occurred when you were six? The fat and skinny are opposites and that may suggest opposing emotions you had at six over an experience. The experience could have been benign, nothing more than a childhood fear {like the boogyman under the bed}. But since you have carried this dream with you for so long it may be more to it. Look back to your earliest memories and see if something comes up.

Jerry