For the first time an animal model has been found for malignant melanoma, opposums with their fur shaved. Research found that even low levels of UV exposure of the infants, so low that the skin didn't redden, was enough to form lesions. These lesions became visible during adolescence and later developed into melanomas in the adult opposums.
... the incidence of malignant melanoma in adulthood can be dramatically reduced by the consistent use of sunscreen in infancy and childhood.
The Texas Biomed researchers found that the application of lotion containing sunscreen to infant opossums led to a 10-fold reduction in pre-melanotic lesions (known to progress to melanoma), in comparison to infant opossums receiving lotion that did not contain sunscreen. This difference in the development of lesions occurred even when low doses of UV light were applied -- so low that they caused no sunburn or even reddening of the skin in the opossums that did not receive sunscreen.
The pre-melanotic lesions did not appear until the infants had become adolescents (equivalent to early teenagers in humans), and prior experiments established that the pre-melanocytic lesions in opossums do not progress to melanomas until the animals are well into adulthood, as typically occurs in humans.
"Based on these results, we speculate that the reason it is particularly important that sunscreens be used consistently in childhood, and especially in infancy, is because skin cells during growth are dividing much more rapidly than in adulthood, and it is during cell division that the cells are most susceptible to UV-induced damage," said VandeBerg. [emphasis mine]