You need to edit this line:

df.loc[df.index[0], 'age'] = df.loc[df.index[-1], 'age']

Complete code:

import pandas as pd
import numpy as np

df = pd.DataFrame({'name': ['tom', 'jon', 'sam', 'jane', 'bob'],
               'age': [np.nan, 25, 18, 26, 17],
               'sex': ['male', 'male', 'male', 'female', 'male']})

df.loc[df.index[0], 'age'] = df.loc[df.index[-1], 'age']

Cheers!!!