Perfectly normal. Armpit hair can be one of the last things to come in during puberty. I had plenty of pubic hair for several years before I got armpit hair. I got my first pubic hair around 12, almost 13, but didn't get my first armpit hair until 15. Nothing noticeable until 16.
Don't worry, it will come in eventually. As will body hair, facial hair, and everything else.
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body hair 
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