Can someone explain this to me?

So my younger brother is 18 and he's a senior. He's graduating this year. I met a new friend who's 18. She graduated from high school last year and she's a freshman in college. We both go to the same college. I'm 20 by the way. So they're both born in 2000. She was born in January and my... show more So my younger brother is 18 and he's a senior. He's graduating this year. I met a new friend who's 18. She graduated from high school last year and she's a freshman in college. We both go to the same college. I'm 20 by the way. So they're both born in 2000. She was born in January and my brother was born in July same year. How come they didn't graduate in the same year? I'm sorry if this sounds really stupid.
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