It depends on the definition of full grown - they don't keep growing physically, but they maintain their puppy like attitude for much longer than just their physical maturity time, it's a boxer trait - they're all young at heart.
You'll notice those spots even in boxers that are flashy - people used to be afraid that a boxers markings on their skin would darken up and they'd look like a dalmation - but that's not true.
Casper is covered with these spots, one of my neighbors was telling me one day that Casper was a dalmation Sharpei mix because there are no white boxers and you can't get a pure-breed dog at the shelter :rolleyes:
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mama to:
-Sumi, lab mix "pound puppy"
-Casper, white boxer rescued from the Dade County Animal shelter