What may be the youngest age a child starts to have some armpit odor?

Andrenarche. Sometimes you will detect a change in the normal sweat/stink profile of your kid as early as 6. This happens when the adrenal glands start increasing some of their hormones at higher levels. It occurs separately from true puberty. If you see any growth acceleration, or other signs that would suggest early puberty, have it evaluated.
Any age. Children sweat just like we do. When they play hard enough, they smell just like any adults. Unless there is hormone imbalance, then that is a different story.