Why has my beard become patchy with white hairs at the age of 27?

Although normal aging and genetics are important to consider here, so is alopecia areata. See a dermatologist.

White beard is caused due to the lack of melanin pigmentation. The causes for premature graying can be outlined as:
-Genetics
-Unhealthy diet
-Excessive alcohol intake & smoking
-Deficiency of vitamins, copper, iodine and minerals. You need sufficient Vitamin B12. Vitamin B12 is required for melanin production.
-Excessive amounts of stress
Make sure that you keep your body healthy and eat foods rich in Vitamins & minerals. If you are graying prematurely, and it doesn't run in your family, or if you have a sudden change in hair color or excessive hair loss, discuss your symptoms and concerns with your doctor.

The patchy appearance of white hair is genetic and it often starts as patchy white islands. You can expect that it will probably continue