While USCIS defines a child under the age of 21 as an Immediate Relative (processing time approx. 1 year) it, inexplicably, defines an unmarried child over the age of 21 as a First Preference Relative (processing time approx. 7 years).
Furthermore, the parents of a US citizen over the age of 21 are defined as Immediate Relatives. Under the current law, if the roles are reversed, the very same relationship has two polar opposite classifications. To illustrate: I am the parent of a child who is unmarried and over 21 years of age and she is considered a First Preference Relative to me. If my daughter were the US citizen, I would be considered an Immediate Relative to her. Under no school of thought that I am aware of does this hold any grain of reason.
The relationship between a parent and a child under the age of 21 and the relationship between a parent and a child over the age of 21 is still just the relationship between a parent and a child. And if the parents of US citizens over the age of 21 are considered Immediate Relatives, then it stands to reason that children of US citizens should be no different.
Please kindly change the category of unmarried child over the age of 21 to immediate relative.

Supporting information:

A  U.S. citizen may file a petition on behalf of his/her:

• Husband, wife, or child under the age of 21 (immediate relative)
• A Parent if the U.S. citizen is at least 21 years of age (immediate relative)
• An unmarried child over the age of 21 and their children (first preference)
• Married child of any age and their children (third preference)
• Brother or sister if the U.S. citizen is at least 21 years old and their spouses and children (fourth preference) http://www.uscis.gov/portal/site/uscis/menuitem.eb1d4c2a3e5b9ac89243c6a7543f6d1a/?vgnextoid=1d383e4d77d73210VgnVCM100000082ca60aRCRD&vgnextchannel=1d383e4d77d73210VgnVCM100000082ca60aRCRD       A  Lawful Permanent Resident can file the petition on behalf of his/her:
• Husband or wife (second preference A)
• Unmarried children under the age of 21 (second preference A)
• Unmarried child over the age of 21 (second preference B). http://www.uscis.gov/portal/site/uscis/menuitem.eb1d4c2a3e5b9ac89243c6a7543f6d1a/?vgnextoid=75783e4d77d73210VgnVCM100000082ca60aRCRD&vgnextchannel=75783e4d77d73210VgnVCM100000082ca60aRCRD