What is the youngest age a child starts to have underarm odor?

No definite age: I don't think there is any definite age described in literature. I can tell that puberty seems to affect the odor of the sweat significantly. The presence of axillary hair seems to contribute to it as well, in that the hair traps and retains sweat and may be a little harder to clean.

See below: Some girls may at 8 years and boys at 10.

About 6: Andrenarche is the point where the adrenal glands begin putting out a higher concentration of hormones, some of which can turn on the odor producing glands of the under arms. It usually preceded the onset of puberty, which is dependent on hormones from the brain. If this is accompanied by increasing growth rate, nipple swelling, darkening of genital hair, the kid should be evaluated by his/her doc