Depends. On baby's exposure to sun light.. A certain amt of sunlight needed for vit. D conversion. But if taking baby to beach use sunblock probable a physical block zn oxide and titanium dioxide .. Stay away from chemical blocks in young baby ( why put chemicals on their skin).
6months. Aap recommends sunscreen for babies over 6 months. This is mainly because babies under 6 months really shouldn't be in the direct sunlight- but if they are, sunscreen should definitely be used! use a broad spectrum sunscreen which protects against uva and uvb light. Along with sunscreen always use sunglasses and a hat as protection as well.
As soon . As soon as you plan on being in the sun, your baby should be covered in sunscreen. Speak to your pediatrician regarding recommendations. Good luck.
Sunblock right away. I would avoid the use of chemical sunscreens and recommend sunblocks such as zinc oxide based products.