Im not sure about what age you can start using insect repellents or sunscreens. But you should look for natural alternatives.

I dont have any children of my own- so this theory is not tested, but I have worked in child care for 9 years, so I have heard alot of theories from parents.
For mozzies: Dabbing lavander oil on children's skin can deter mozzies. The lavander oil is much more natural then other insect sprays- plus it is not being sprayed into the air therefore not being breathed in.

As for sunscreens: I dont like them myself. The smell, the oilyness, Yuck! A friend of mine uses the Cancer Council sunscreen on her babies.And she is the same as me.

Well I hope this helps, good luck!