Will I have melanoma when I am older? Or is it completely impossible to predict?

Immpossible to know. There is no way to tell what will happen in the future. All we can asses is the risk of these things happeneing. But, a person can take every concievable risk, and still not have the outcome.
Can't predict. You can't predict at what age you will have a higher chance of developing melanoma. This will depend on multiple risk factors such as family history, sun exposure, tanning bed, ect... However, in general, in people over 15, the incidence steadily rises with age. It is now one of the most common types of cancer in people between the ages of 15 to 34. The highest incidence is in people over 80.
Melanoma. Risk for melanoma is mostly related to genetics and uv light exposure. Lifetime risk depend on your race, where you grew up and sun/tanning history.