It can be due to fungal infection(pityriasis versicolor- in which fungus causes white spots which join to form white patch) or due to seborrheic dermatitis.
But since you r telling it's present for a very long time, it can be Nevus depigmentosus or Nevus anemicus(which are birth marks, might not have been so prominent at birth, but noticed later). These birth marks are harmless.
But if it's due to fungal infection or others, it needs to be treated
Next Steps
consult now with me online/any dermatologist for proper diagnosis and follow up
Health Tips
do not apply any cream by yourself/from medical store, as it might cause contact dermatitis or if they have topical steroids it might damage your skin causing atrophy and thinning of skin, increasing the problem.