3 - 4 weeks if it's not with mum (eg: An abandoned or orphaned kitten). Otherwise you should really wait till 6 or 8 weeks. Use very small amount of mild pet shampoo. Rinse extremely well and most importantly, dry immediately and dry well! They can't regulate their body temp so even in summer, important to dry well and quickly. Don't use flea treatments until 6 weeks of age.
If the kitten has not been weaned from it's mom yet let her do it, but if they are older a light bath in vinegar water is safe...