II. Fill in the Blanks (0 points per question, total 30 points)
7. (10 points) If the units and tens digits of Grandpa's age are swapped, it becomes Dad's age. The age difference between Grandpa and Dad is 5 times Xiaolin's age. Therefore, Xiaolin's age is $\qquad$ years.
【Analysis】Let Grandpa's age be $\overline{\mathrm{ab}}=10 a+b$, then Dad's age is $\overline{\mathrm{ba}}=10 b+a$. According to "the age difference between Grandpa and Dad is 5 times Xiaolin's age," we get $10 a+b-(10 b+a)=9(a-b)$, so $9(a-b)$ is a multiple of 5. Since the value of $a-b$ can only be less than 10, we can deduce Xiaolin's age.

【Solution】Solution: Let Grandpa's age be $\overline{\mathrm{ab}}=10 a+b$, then Dad's age is $\overline{\mathrm{ba}}=10 b+a$. The age difference between Grandpa and Dad is: $10 a+b-(10 b+a)=9(a-b)$,

Since the age difference between Grandpa and Dad is 5 times Xiaolin's age, $9(a-b)$ is a multiple of 5, which means $(a-b)$ is a multiple of 5,

Since $a-b<10$, $a-b=5$, so Xiaolin's age can only be 9 years old.

Answer: Xiaolin's age is 9 years old.
Therefore, the answer is: 9.