6. (3 points) Zhang Bing was born in 1953. In a certain year before this year, his age was a multiple of 9 and equal to the sum of the digits of that year. How old was he $\qquad$ that year.
【Analysis】According to the problem, let that year be $19 A B$, then his age in that year is $1900+10 A+B-1953$ years old, which is also $1+9+A+B$ years old. Since his age is a multiple of 9, then $1+9+A+B$ is a multiple of 9. We can then set up the equation and solve it further.

【Solution】Let that year be $19 A B$; according to the problem, we have:
$$
\begin{array}{c}
1900+10 A+B-1953=1+9+A+B, \\
10 A+B-53=10+A+B, \\
9 A=63, \\
A=7 ;
\end{array}
$$

Since his age is a multiple of 9, then $1+9+A+B$ is a multiple of 9;
$1+9+7+B=17+B$ is a multiple of 9;
When $B=1$, $17+1=18$ is a multiple of 9;
Therefore, in the year 1971, his age is a multiple of 9 and equals the sum of the digits of that year; his age that year is: $1971-1953=18$ (years old).

Answer: That year he was 18 years old.
The answer is: 18.