Normally melanin gets synthesised and homogeneously spread through sun exposure to get an even tanning. If this process gets interrupted the melanin is focusing on very sensitive areas which are normally uncovered and is creating dark spots: therefore the face and the hands are the first victims of the sun.
Some people, with the genetic predisposition, tend to get pigment spots before their forties; other people get it from medication like psychotropic drugs and antibiotics. A hormonal imbalance can also cause pigment spots: during a pregnancy or caused by the birth control pill. Sythetic parfums, perfumed products and hairspray can also cause pigment spots.
After 30 years the active melanocytes decrease by 10% to 20% each decade. Nowadays we know that UVB and especially UVA intensify the disorder of pigmentation.