I was employed for a year, then left for a summer (3-month) internship for my college degree with intentions of returning to the company. When I went back and talked to the manager i was informed that Corporate would not allow them to hire me because they don't want to employ too many 18 year olds (or older). Is this covered in any anti-discrimination laws either federal or state? (Michigan)
The ADEA (Aged Discrimination Employment Act) covers only those 40 years of age and older. Is this discriminatory? Certainly can be especially if it can be proven there is an illegal hidden motive which is not easy to prove. The EEOC (Equal Employment Opportunity Commission) does not cover this (they cover The Civil Rights Act of 1964 - Title VII.) You can by all means try to find an attorney who would take on the case, but in all honesty it is not worth it. Move on.
No. Age discrimination laws only protect older workers (individuals over 40. It may very well be discrimination not to hire an 18 year old, but it is not illegal in any state of the US.

The Age Discrimination in Employment Act of 1967
http://www.eeoc.gov/policy/adea.html
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age discrimination only applies to those over the age of 40, they are free discriminate against you because of your age