If a baby was never exposed to sun until 5?

They probably wouldn't survive to age 5 unless you artificially supplemented vitamin D. Even if you did, you would probably encounter other problems we don't even know about. Sunlight is not harmful, excessive sunlight can be. People are healthiest with modest, regular sunlight exposure. Severe sunburn is definitely harmful, especially during puberty. Take precautions against sunburn, but sunlight paranoia is unwarranted.