What is the mental age of someone with mild retardation?

You need to have him or her tested, and ask the psychologist who did the testing.
Also, it depends on how old the person is. E.g. a 10 year old with "mild MR" will have a different mental age than a 20 year old with "mild MR."

they are usually born this way===its us that takes differenttimes to know when our child iss like this

There is no one answer. It depends on the extent of the retardation. It takes testing by pro's to determine this.

Most of the MR patients I have worked with tend to have a mental age of around 7. This is based on factors such as how they interact with people and the enviroment, their behavior, their hobbies, etc.

Depends on the person, and the severity of the mental problems...my brother is almost 30, but at times, hes like 12., and sumtimes, hes like 3-4, it really depends on the situation hes in as well, then other times, hes totally 30.

It totally depends. There are several tests that you can submit anyone with mental deficits/defects to in order to determine "mental age". The oldest and most trusted is the "Standard Intelligent Quotient Test", IQ test. The lowest "functional" adult score is about 80, the highest known score is 200. This belongs to Stephen Hawking. I believe that what is considered "Mild Retardation" starts at an IQ of 75. This would be the IQ of an average 16 year old. However what is socially considered "mental age" has more to do with perceived innocence and interests than IQ. A 32 year old, with an IQ of 135 (borderline genius) can be socially inept and come across more like a teenager or younger. While a person with Downs Sendrom at age 19 with an IQ of 65 could seem worldly and wise.