My 15-year-old Cat, Who Has Been Gray All Her Life,

My 15-year-old cat, who has been gray all her life, now has brown hair all down her back. Her face and legs and underside are the same gray and white as always. For 14 years she was an indoor cat, but for this last year she has been outdoors. Is there something to be concerned about? Or is this normal for an aging cat?