7. (6 points) Xiao Ming is 12 years old this year, and his father is 40 years old. When Xiao Ming is $\qquad$ _ years old, his father's age will be 5 times Xiao Ming's age.
【Analysis】According to the problem, the age difference between the father and the son (40-12) remains constant. Then, based on the fact that the father's age is 5 times the son's age, the age problem is transformed into a difference-multiple problem. Therefore, the son's age can be found when the father's age is 5 times the son's age.

【Solution】Solution: $(40-12) \div(5-1)$, $=28 \div 4$, $=7$ (years old), Answer: When Xiao Ming is 7 years old, the father's age is 5 times Xiao Ming's age. Hence, the answer is: 7.
【Comment】The key to solving this problem is that the age difference between the father and the son does not change no matter how many years pass. By using the difference-multiple formula, the son's age can be found when the father's age is 5 times the son's age.