Two fawn parents *cannot* produce a brindle puppy. Two brindle parents, on the other hand, certainly can produce a fawn puppy if they're each carrying a 'hidden' fawn gene.
Fawn puppies often have dark or black 'guard hairs' (I call them) along the spine, which typically fall out as the puppy matures; dark fawns sometimes have 'smudging' on other parts of the body as well, which may or may not stay into adulthood. Pigment continues to migrate after birth, so the white areas will shrink with age, and brindling spreads out. As to whether the base color itself gets darker or lighter - honestly, in my experience it can go either way, but I do think typically it either darkens a bit or doesn't really change.
Yorkies do get lighter as they age (going from black to silver) - the same happens to Kerry Blue Terriers - but that is due to a specific gene which the Boxer does not have.