The canonical Bell Biker helmet that I remember was an ugly and expensive white bowl-thing for serious road cyclists that started becoming popular in the late 70s as ten-speed bicycles became more popular with adults.

https://americanhistory.si.edu/collections/object/nmah_11831...

I think I bought my Bell Biker helmet around 1980. The stylish kid-sized helmets that became widely available during the 80s did not exist in the 70s.