Let's solve the riddle step by step.
We have the equation:
\[ \frac{x}{4} + 6 = \frac{x}{8} + 8 \]
where \( x \) is the dog's age.
First, we can eliminate the fractions by multiplying the entire equation by 8 (the least common multiple of 4 and 8):
\[ 8 \left(\frac{x}{4}\right) + 8 \cdot 6 = 8 \left(\frac{x}{8}\right) + 8 \cdot 8 \]
This simplifies to:
\[ 2x + 48 = x + 64 \]
Now, we will move \( x \) to the left side and the constant terms to the right side:
\[ 2x - x = 64 - 48 \]
This simplifies to:
\[ x = 16 \]
So, the dog's age is \( 16 \) years old.
Therefore, the answer is:
16 years old.