I think that would be more confusing. Though 58 inclusive or exclusive?
See, in many parts of the world, when you ask someone how old they are, there are different ways of counting vis-a-vis that 0 year.
My Dad, Hungarian, when someone asked his age, would always "add one" when compared to the US way of thinking about it. When he was, say, 59 by US standards, he would say he's 60 when asked. I asked him about it, and he would respond, "I'm in my 60th year." For them, they counted 0-1 as Year 1 of life.
So Canon Law saying "not yet begun their 60th year" according to the same mindset as my Dad. You COMPLETE your first year when you're 1 year old. You BEGIN your 2nd year when you're 1 (by our standards). So you always have to +1 from US standards.
So someone who is (by US standards) 59 is no longer obliged to fast.
This actually causes some confusion among computer programmers, by the way. Some programmers count collections beginning with #1, and others 0. In most computer languages, the first element of a collection starts with 0. So if you have a list of 10 items, they would have the index or number from 0 to 9.
Canon Law, as per usual, tries to be very precise. They couldn't just say, "you have to fast until you're 59." Until 59 inclusive or exclusive? What does it mean to be "59"? So Canon Law defines it as having completed your 59th year, which would happen on your 59th birthday.
I realize that, but in vernacular American English, if you say "I am X years old", or "I am X", it means "I have had my Xth birthday".
Jone's manual was supposedly "Englished" and adapted to the customs of the United States, per the title page. Phrasing it in the way that Americans generally speak and understand would seem to be the best way of doing that.
Don't even get me started on his deeply troubling comments about inchoate commission of
peccatum illud horribile inter christianos non nominandum within marriage. That passage totally destroys one's credibility when trying to use Jone as a reliable guide to traditional Catholic moral theology, and one's listener is both aware of it
and dislikes traditional Catholic moral theology to begin with. I really, really want to believe that something got lost in translation.