Options for: "with an average age of 12 (the youngest being 11 and the oldest, 14)."

While working on a translation, I came across this type of construction.

How do you construct/punctuate this?

"with an average age of 12 (the youngest being 11 and the oldest, 14)."

This version is how I would actually say it, but is the punctuation be correct?

A couple of other options that might work:

(youngest, 11; oldest, 14)
(the youngest aged 11 and the oldest aged 14)

Are there any specific rules for this?