if you are a 40 year old man 15% body fat is fantastic in the USA
you would be hard pressed to find anyone who was that lean these days unless they were crackheads, just go to a ball game or a sports bar and look at the physiques of a normal american man that age. you won't find many who are even under 20%!
BUT if you want to look good, have a lean stomach and have some abs showing you need to get closer to 10%. it boils down to your goals