Suppose we have a
an old line of text
You can use
datefrom=$(git log --pretty=format:'%cd' --date=format:'%Y-%m-%d' -S 'line to find' -- file1.txt)
That will result in date of commit that introduced that string.
echo $(( (`date +%s` - `date +%s -d $(datefrom)`) / 86400 ))
which results in number of days from introducing commit.
Ofcourse you can put everything in one command and make an alias, or you can create git-command-name script and put it into user/bin folder and git will recognize it as git command that you can invoke with
git command-name 'line to find'