Here's a link to an article from Acoustic Guitar called "Why Do Guitars Sound Better As They Age?" It was written by Richard Johnston, co-owner of Gryphon Stringed Instruments.
http://www.acguitar.com/article/default.aspx?articleid=24089
While the article is specifically about guitars, it would seem that some aspects of how guitars age -- specifically, the effect of moisture on wood over time -- would be applicable to ukuleles.
http://www.acguitar.com/article/default.aspx?articleid=24089
While the article is specifically about guitars, it would seem that some aspects of how guitars age -- specifically, the effect of moisture on wood over time -- would be applicable to ukuleles.