The total age of the 5 children is \( 5 \times 8 = 40 \) years. When the father's age is included, the average age becomes 13 years for 6 people:
\[
\text{Total age of 6 people} = 6 \times 13 = 78 \text{ years}.
\]
Thus, the father's age is:
\[
\text{Father's age} = 78 - 40 = 38 \text{ years}.
\]