a late-developing letter in English. Called ipsilon in German, upsilon in Greek, the English pronunciation of the name, "wy" (spelled out as wye by 1857) is of obscure origin.
The sound at the beginning of yard, yes, yield, etc. is from Old English words with initial g- as in got and y- as in yet, which were considered the same sound and often transcribed as Ȝ, known as yogh. The system was altered by French scribes, who brought over the continental use of -g- and from the early 1200s used -y- and sometimes -gh- to replace Ȝ.
The French also tended to substitute -y- for -i-, especially before -u-, -n-, or -m-, or at the end of words, to avoid confusion in reading (see U), which might also partly explain this tendency in later Middle English.
In reference to anything of the shape of a letter Y, by 1510s. As short for YMCA, and similar organization names, by 1915. YA for young adult as a publishing market is by 1974. Y-chromosome is by 1909, based on x-chromosome.