Just remember, my beloved Gracie - the 3/4 Great Dane, 1/4 Lab mix started going gray - white actually, at about age 4. That, though, apparently meant NOTHING. By the time she was 7 or so, her (formerly solid black) muzzle was all white and she had white rings around here eyes. And yet she lived to the ripe old age of 14 1/2 - almost unheard of for Great Danes - and the vet (who called her "the wee lassie" - he was Scottish) attributed it solely to her being "spoiled rotten" - which is something Simba has in common with her! And as he's a smaller breed, he's expected to live a longer time anyway!