Every person with this condition I've seen so far looked much older than their actual age. Characteristics incluide male pattern baldness, big bone structure, tall stature, chubby etc...

I though a lack of testosterone meant having an androgynous appearance, which would make you look younger. People with kleinfelter's don't appear very androgynous, at least not the ones I've seen.

Is it something to do with the medication they take i.e. testosterone suppliments? If so does this affect body builders too?

What you're describing is rare. It could be more stress in life takes a toll. However Klinfelter's (Kleinfelter's) can be a mosaic occurrence and other genetic variances are in effect. External meds given by a physician to attempt to correct the situation will also play an effect.

yeah, I sort of noticed that too. The people with klineteftlers I've seen don't appear any more andro than a regular guy their age.

Unless you had a karyotype of your genes many people with Klienfelter's never get diagnosed with it.  The generalities of being tall ect are more for extreme cases and can also be a sign of other genitic conditions. As many as 1 in every 500 males can be XXY but most wouldn't present with any symtoms of the condition.. here a link that explains it in detail. 

http://en.m.wikipedia.org/wiki/Klinefelter's_syndrome

Androgen Insensitivity (AI) could be the reason for the tallness and aging since the body dosen't react much to testosterone and with out some 'sex' hormone the body can't regulate it self well, muscle would be hard to maintain, and since there's little to no estrogen in the system to take it's place the body just dosen't know what to do.

While never officially diagnosed, my wife's first impression of my not so awesome nakedness was Klienfelter's. After looking it up I have to agree, the photos can very well be me.

With that said, my nickname growing up, and through h/s and beyond was Baby Face. Up untill I was hitting 25, I always got carded. Sadly by that time the MPB that started when I was 14 or so hit big time. Lack of hair certainly gives the impression of being much older. Health, energy and attitude wise I'll hold my own against those much younger. My wife is constantly admonishing me; "You're NOT 26 anymore"