From my memory. I recall a case from a patient who was around 60. Late-onset Wilson's is not unknown. There's a Slavic gene that tends to present late. http://www.ncbi.nlm.nih.gov/pubmed/17433323.
Old. Wilson disease may have partial inheritance, incomplete penetrance, and variation in the genetics of the disease, so that the only way a patient knows they have it, is when they are tested (often during work-up of a relative). Therefore, there is not really an age limit. However, most come to attention in early twenties to thirties.