Q: what is the lowest bodyfat percentage for an average person can acheive (man), who trains hard and regularly at the gym following a good diet?

A: It's going to vary depending on the person's age, past health and weight history, and genetics. The latter especially plays a very big part and can vary quite a bit. I'd say most men under 25 should be able to reach 10% body fat (seeing the six-pack). A few will be able to drop as low as 6-7%. But 10% is a good target that's achievable by most younger men. For older men it becomes more and more difficult with age, especially if they start out of shape.