Are 1984 babies the same generations as babies born in 1996 because they’re all in the millennial category

I mean someone born in 1984 is 12 years older than someone born in 1996. How would they even be in the same generation? They’re childhood and high school years are different.
1984 babies graduated from high school in 2002/2003 depending on their birthdays and 1996 babies graduated in 2014/2015 depending on their birthdays. Those are two different times. Just saying.