In a small town, the ages of residents are grouped into five categories. $20\%$ of residents are in the $0-10$ age group (represented by $5$ years), $20\%$ are in the $11-20$ age group (represented by $15$ years), $20\%$ are in the $21-30$ age group (represented by $25$ years), $20\%$ are in the $31-40$ age group (represented by $35$ years), and the remaining $20\%$ are in the $41-50$ age group (represented by $45$ years). What is the difference between the mean and the median age of residents in this town?