You know, I like bewbs as much as the next heterosexual male, but I'm all for being different with the dragonborn and any other non-mammalian species in D&D. If the concept of the species does not suggest lactation, then there should be no breasts or nipples of any sort.
If you want to include some other outward signal of a creature's fitness/sexual maturity (male or female) then figure out the species's ecological niche, think about its evolution, and come up with something else that makes sense. Maybe a color change from better camouflaged adolescent to richer colored adult, different texture to the skin/scales, or a noticeable scent.