I answered a question a few months ago that may be of interest to you. It deals specifically with those who died from battle wounds but I think the discussion of variability contained in the answer is especially relevant here.

To sum up a little: Going off the info in the Prose Edda anyone who died from sickness or old age became a ward of the goddess Hel. There is nothing in the mythological texts that I am aware of that deals specifically with those that died young though. Having said that, there is evidence for a large amount of variance in beliefs so we can't really take the mythological works at face value. If you have any questions after reading my other post go ahead and ask them here.