Oct 13 (Reuters) - Johnson & Johnson said on Friday it has received an expanded U.S. approval for its blockbuster psoriasis drug Stelara to treat adolescent patients aged 12 and over with moderate to severe cases of the unsightly skin condition.

Stelara was already approved to treat adults with the condition, a chronic, autoimmune inflammatory disorder that causes inflamed, scaly, sometimes painful skin patches.

It is also approved to treat adults with the related condition psoriatic arthritis, and for moderate to severe Crohn's disease.

About one-third of people who develop plaque psoriasis do so before 20 years of age, J&J said.

"Psoriasis is a highly visible disease, and it is essential that these younger patients and their caregivers have options that can effectively reduce the difficult-to-conceal and often misunderstood plaques," Michael Siegel of the National Psoriasis Foundation said in a statement.

Stelara is one of J&J's most important pharmaceutical products, with 2017 sales expected to reach $3.7 billion and increasing to more than $5 billion by 2021, according to Thomson Reuters data. (Reporting by Bill Berkrot; Editing by Jonathan Oatis)