My 6 year old daughter's armpits is smelling; she is very active but I don't think that it has to smell at a very early age as hers; it has been smelling from the time she was 5 years of age or before that; can't remember exactly when it started?

It happens. This can be normal body germs that colonize the area. It can also be the common odor glands of her body. These are under the control of the adrenal glands and are not necessarily part of any early puberty thing, it can occur that young.
What kind of smell? If it is fishy or unusual, you should have her checked by a pediatrician for a metabolic abnormality. If it is sour or smells like adult sweat, be sure she is very clean and perhaps apply some rubbing alcohol to this area each day to help keep it dry and discourage bacterial growth.