Age alone denotes vintage in the world of musical instruments. An example from eBay can be found in the category "Vintage (Pre-1980)"! By THAT definition, there are quite a few of our fellow UU'ers who are "vintage"! When it comes to ukulele's, the desirability of a particular vintage instrument will be a combination of the maker (Martin, Nunes, Dias, Santo, KUMALAE, Kamaka, etc.), the year that the instrument was made, the quantity originally manufactured & that survive today (thus the relative scarcity), and of course, current condition.
Like any collectible, a vintage ukulele may bring drastically fluctuating prices & in general, they are enjoying the current upswing in the ukulele's popularity - so for those of us who are vintage & find vintage ukes impossible to resist, the price we might have to pay for a particular make/model/year/condition is rising quickly!
Hope this helps! Mahalo!