Terms to describe age groups

I have three age groups that I want to distinguish in my research project.

They are as follows:

• 18-45 years - I have called this group young adults
• 46-65 years - I have called this group adults
• 66 years and over - I have called this group seniors

Is there a better way (one word) to describe the 18-45 age group? I don't like the term young adults. (Adults and seniors work fine for me!)

Solution 1:

How about just adults? I would recommend a breakdown like this:

• 18-45 years - adults
• 46-65 years - middle-aged
• 66 years and over - seniors

Solution 2:

What about prime adult or early adult? From the Wikipedia entry for 'young adult':

A young/prime adult, according to Erik Erikson's stages of human development, is generally a person between the age of 20 - 40, whereas an adolescent is a person between the age of 13 - 19,1[2] although definitions and opinions vary. The young adult stage in human development precedes middle adulthood. A person in the middle adulthood stage is between the ages of 40 - 65. In maturity, a person is 65 years old or older.