Assign string values based on multiple ranges

df['new_col'] = pd.cut(df['existing_col'],
                             bins=[1, 5, 12, 18, 24, 30, np.inf],  # bins = ranges ie 1, 5 = 1 to 5
                             labels=['1 to 5', '5 to 12', '12 to 18', # labels map to the bin ranges ie: 1, 5 = '1 to 5'
                                     '18 to 24', '24 to 30', '>30'],
                             right=False)
# If you're going to use cut use np.inf not an arbitrary upper bound value of 999999. OP's function is also lowerbound  inclusive so need right=False