Vocaloid Big Al would be considered the eldest (oldest). His official age is 25 (twenty five). While Vocaloid Ueki-loid currently has no information on his age therefore he can not be viewed as legitimately being the oldest.
If this question is referring to 'oldest' in terms of product release, then it would be the English Vocaloids Leon and Lola. They were both released in January of 2004.
- See also