Why is the younger generation shorter?

I am 16 and 5'7 ( 170-171 cm )... I remember I used to be 4'7--4'8 in 5th grade and 4'9-4'11 in 6th grade but nowadays most 5th and 6th graders r midgets like they r same height as 3rd graders... even 8th graders r midgets like 4'7... I was 5'2-5'4 when I was in 8th