Q: why will my i mac not mirror image to my apple tv

My Imac with mirror imaging turned on will not show on my apple tv

Processor  3.2 GHz Intel Core i3

Memory 4Gb 1333 MHz DDR3

Graphics ATi Radeon HD 5670 512 MB

I bought my apple tv at christmas because I wanted to be to watch what I see on my Imac screen through my apple tv on my television but I seem not to be able too.

The only thing I have read is tha my mac being nearly 3 years old may not be a god enough spec