This is an authentic dark navy blue service dress blue middy jumper shirt with white striping on the collar and cuffs, and white embroidered stars on the sailor collar from the USN.
100% wool serge.
Has the white cotton identification tag sewn on under the collar: the writing on it has faded so badly it is unable to be read, though; we've included an image of it for you.
Since the uniform has not changed much over the years, and, as there were 5 different ships that were named U.S.S. Yorktown (the first in 1839, the last was commissioned in 1984, which is now awaiting scrapping) (thanks to Wikipedia)it is difficult to determine the age of this for certain.
In excellent condition. There must have been some medals attached at one time, as there are pin holes where they would have been attached: we've included an image of that for you, as well.
Final photograph from the 1940s courtesy of Sheaf Ephemera who has a ton of fantastic vintage imagery. Thank you, Dick!