Melasma does not show up evenly across everyone. This page names the groups the sources say are more likely to develop it.
Women between 20 and 40 lead the list
DermNet says melasma is more common in women than in men, with an onset typically between the ages of 20 and 40 years.
That makes sex and age the first two things to check against your own situation.
Skin tone also matters
Medical Xpress reports that patients with darker skin also are more likely to develop melasma.
So the picture has three parts: being a woman, being in that age window, and having darker skin. Having one of them is not a diagnosis.
Background on melasma from three sources
The MedlinePlus entry on melasma and the AAD's melasma overview cover the condition from the start. DermNet's page on melasma as facial pigmentation gives a clinical view.
If new dark patches appear
If you see new dark patches on your face, have a provider look at them. Naming the condition correctly comes before any treatment plan.
Age as the factor to watch
The answer changes with age. The sources place the typical onset between 20 and 40 years, so that is the stretch of life to watch most closely.