Creating a script with options to access different directories and files

If I was implementing this in bash, I'd do the following. I won't comment on it much: feel free to ask specific questions though -- check the bash man page first if you don't know how a particular command works.

#!/bin/bash

# read the population from all the files
# map the filename to it's population figure
declare -A population
while IFS= read -d '' -r filename; do
    population["$filename"]=$(tail -1 "$filename")
done < <(find . -type f -print0)

# prompt the user for the year
read -rp "What year? " year

# find the relevant files for that year
year_files=()
for filename in "${!population[@]}"; do
    [[ $filename == ./"$year"/* ]] && year_files+=("$filename")
done
if [[ "${#year_files[@]}" -eq 0 ]]; then
    echo "No files for year '$year'"
    exit 1
fi

PS3="Select a function to calculate: "
select func in minimum maximum average quit; do
    case $func in
        minimum)
            min=${population[${year_files[0]}]}
            for file in "${year_files[@]}"; do
                if (( min > ${population[$file]} )); then
                    min=${population[$file]}
                fi
            done
            echo "Minimum for $year is $min"
            ;;
        maximum)
            max=${population[${year_files[0]}]}
            for file in "${year_files[@]}"; do
                if (( max < ${population[$file]} )); then
                    max=${population[$file]}
                fi
            done
            echo "Maximum for $year is $max"
            ;;
        average)
            count=0 sum=0
            for file in "${year_files[@]}"; do
                (( sum += ${population[$file]} ))
                (( count++ ))
            done
            echo "Average for $year is $(( sum / count ))"
            ;;
        quit) exit ;;
    esac
done

I write a simple awk script which does the same as what you are doing:

# read 'year' & 'option' from user
# or you can pass as argument to the command $1<-->$year & $2<-->$option

find /path/to/$year -type f -exec \
    awk -v select=$option '
        FNR==4 { sum+=$0; avg=sum/++count; 
                 max=(max>=$0?max:$0);
                 if (count==1) min=$0;
        }
        count>1 { min=(min<=$0?min:$0);
        }
    END{ stats=min","max","avg","min"\n"max"\n"avg;
         split(stats, to_print,",");
         print to_print[select];
    }' {} +

Explanation inline:

# read 'year' & 'option' from user
# or you can pass as argument to the command $1<-->$year & $2<-->$option

find /path/to/$year -type f -exec \
# find all files under "/path/to/$year". $year will be substitute with the value 
# of 'year' variable read from user-input or replace it with '$1' as first argument to the command

    awk -v select=$option '
    # read the value of shell 'option' variable into an awk 'select' variable 
    # replace with '$2' as argument to the command

        FNR==4 { sum+=$0; avg=sum/++count;
        # if it's 4th line of each input file, sum-up the value into 'sum' variable
        # and calculate the 'avg' too when 'count' will increment once each 4th record in a file is read

                 max=(max>=$0?max:$0);
                 # its a Ternary operator (condition?if-true:if-false) and finding maximum value

                 if (count==1) min=$0;
                 # keep the first file's 4th line's value as minimum. you could use `NR==4` instead
        }
        count>1 { min=(min<=$0?min:$0);
        # same as max, update the 'min' if value in current file is smaller than 'min' in previous file
        }
    END{ stats=min","max","avg","min"\n"max"\n"avg;
    # saving all variables' value into single variable with comma separated. I used <min"\n"max"\n"avg> as 
    # fourth element which we will use it as "All" option that each separated with newlines.

         split(stats, to_print, ",");
         # building an array called 'to_print' from 'stats' variable above with comma separator to distinguish 
         # the elements from each other.

         print to_print[select];
         # this will print the element which user-input as an option.
         # if input 1: will print 'min'
         # if input 2: will print 'max'
         # if input 3: will print 'avg'
         # if input 4: will print 'min' \n 'max' '\n' avg
    }' {} +