The calf grew into a cow at about the rate that Milo grew  into a man.

Calves grow for a year or so.

  A rather freakish man apparently, since grown cows can weigh over 1000 lb.

Also freakish since he had the maturation rate of a cow, or had a knack for breeding cows with the maturation rate of a man.

  The point is, the calf grew old along with the boy.

No, it didn't.

  I have been pondering this story for a couple of years...

Have you really been pondering this story for a couple years? Maybe not that closely.

Your metaphor is broken. Milo lifted lots of different calves.

It's a Greek myth, so how much realism do you really expect? For what it's worth, the way I've always heard it was that the boy and the calf did grow up together.

Seriously: your only problem with the story is that the cow and the boy grew up at about the same rate? Really?

Milo of Croton was a real guy, so it's not a "Greek Myth" in the usual sense. Almost certainly most of the exaggerated stories told about him are apocryphal, though.

Legends say he carried his own bronze statue to its place at Olympia, and once carried a four-year-old bull on his shoulders before slaughtering, roasting, and devouring it in one day.

That's not the point and you know it.

http://www.douglasadams.com/dna/pedants.html

It might not be a perfect metaphor but it does introduce the concept nicely. I have no reason to doubt his claim that he's pondered it for a couple of years.

Wow, what an amazing reading comprehension fail. Here's the text from the article:

The story goes that Milo, a famous wrestler in ancient Greece, gained his immense strength by lifting a newborn calf one day when he was a boy, and then lifting it every day as it grew. In a few years, he was able to lift the grown cow.