Any idea on what causes spots on arm, and ways to prevent it from happening? These spots is in the skin, but not lumps.

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When did you first notice these spots? Any other body areas exhibiting spots like these, or larger (at birth [birthmarks] or new)?

About a few months already. These spots are similar to the ones the elderly have. Not itchy or anything, no lumps. Is just on the skin.

Age spots (solar lentigo) are hence named because, well, they are noticed much more in the elderly after decades of long exposure to UV.

Unless you're telling me you're working out in the hot sun for days on end for the past few months with no sunscreen (and having it appear only on one arm? and not all around on the other side of the arm?), they might not be lentigo [since there's only so much detail one could see from a phone cam pic].

Looks like pigmentation. You're out under the sun a lot?