::The article and the states that adult giraffes are mostly invulnerable to predators but lions may be able to kill them if they can get them to fall over. The article also states that giraffes of any age are commonly preyed on by lions in Kruger. The lion article also states that lions usually avoid adult giraffe but regulary hunt them in Kruger. 

 Anyway. the exact qoute from Estes is: Great size, superior vision (day and night), speed and formidable hooves make grown giraffes largely invulnerable to predators–although lions are able to kill even bull if they can get them down and secure a throat or nose hold.