I feel like this kind of operation on a list feels more naturally expressed by filtering the list and taking the length of the filtered list.

Like this line of JS feels so much easier to read than that line of python:

    ages.filter(age => age > 17).length

Directly translating this approach to python:

    len(list(filter(lambda age: (age > 17), ages)))

Although a better way to write this in python I guess would be using list comprehensions:

    len([age for age in ages if age > 17])

which I feel is more readable (but less efficient) than the APL inspired approach. Overall, none of these python versions seem as readable to me as my JS one liner. Obviously if the function is on a hot path iterating and summing with a number is far more efficient versus filtering. In that case i'd probably still use something like reduce instead of summing booleans because the code would be more similar to other instances where you need to process a list to produce a scalar value but need to do something more complex than simply adding.