Filling created feature with values
If I understood your question right - 
you've created a new feature age&lt;55 - which gets True/False, hence, there is no need for the second feature since it has a perfect (negative) correlation with the first one (i.e False in every record that the other new feature got True..).

2 last notes:


Note that the second feature ain't addeing any new information to the first one.
Note that you've missed the recodes with age==55


I would suggest:

df['is_old'] = df['age'].apply(lambda x: x&gt;55)