input: Write a python function to filter a dataframe by age column.
  assert filter_df_by_age(df, 30).equals(expected_df_30)
  assert filter_df_by_age(df, 25).equals(expected_df_25)
  assert filter_df_by_age(df, 40).equals(expected_df_40)
output: def filter_df_by_age(df, age):
      return df[df['age'] == age]