I'd say at least old enough to have a grounding in as many of the major sources as possible. Half the fun of UF is watching how things you never thought could mesh go together seamlessly. The next half is learning about the sources you didn't know. The third half is just the reading experience overall -- so I'd say they should be old enough to handle "grown up" writing.

-- Bob
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My race is pacifist and does not believe in war. We kill only out of personal spite.