You can't undo damage that's already done. But no, childhood sunburn, while it increases the chance of developing skin cancer, doesn't mean your fate is sealed. If anything, it gives you more reason to minimize sun exposure. Continued exposure definitely has an impact on whether or not early skin damage eventually results in serious problems. Certain antibiotics, diuretics, and some topical acne medications such as tretinoin are among the many prescription drugs that can cause increased sensitivity to the sun.
Reactions can range from a bad sunburn to a rash that looks like poison ivy.
Such reactions aren't dangerous and it's important not to discontinue any medication without checking with your doctor. Any time you will be exposed to the sun either directly or indirectly through windows or reflections, it makes sense to wear sunscreen. For an all-day outing like a trip to the zoo, reapply every 4 hours. For a day of skiing, every 2 to 3 hours would be wiser. Liberally and often if you're outdoors a lot.