7 or 8. At puberty kids will need deodorant. Some children may need it earlier than that. Antibacterial soap may also be helpful. If there are signs of puberty (breasts, pubic hair, rapid growth) under age 8 in girls the patient should see their pediatrician. If there are signs of puberty (testicles larger, pubic hair, acne, rapid growth), under age 9 in boys then patient should see pediatrician.
When they stink. The adrenal glands can begin triggering the odor generating glands well before true puberty & this can be as early as 6. Since peer pressure is as much or more an issue in school age kids as it is for us, you don't want them to stand out in a way they get picked on.When you notice the issue it is time to deal with it. Briefly let them know this is part of life.