Multiple Columns to calculate new column using equation

Hi! I have a very large dataset and would like to calculate number of members in the household who are considered children ( aged under 18). I have a column for number of household members, as well as 11 columns for each household member's age. My initial thought would be to select those who are under 18 and subtract from total household members. I've tried a few different lines of code unsuccessfully and I'm not sure how best to go about executing this. Any help is greatly appreciated!

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