The mathematical operation syntax for bash is $(( )). I also recommend $() for command nesting instead of using the ` backticks
echo "$(( $(date +%s) - $(stat -c %Z howoldisthisfile.txt) ))"
You do realize that on posix filesystems there is no "created time" flag, only a "last modified" one, I hope. It's impossible to know with certainty how old a file actually is, only how long it's been since it was last changed.