6. 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, so that year he $\qquad$ years old.
【Answer】 18

【Analysis】The sum of the digits of this year is a multiple of 9, which can only be 18 or 27, meaning Zhang Bing was 18 or 27 years old that year. If Zhang Bing was 18 years old, then this year would be $1953+18=1971$, and $1+9+7+1=18$ meets the requirement; if Zhang Bing was 27 years old, then this year would be $1953+27=1980$, and $1+9+8+0=18$ does not meet the requirement;