Hi Onell,

This is not a run-on sentence.

In societies where members consume little salt, their blood pressure typically does not rise with age.
"where" marks the presence of a dependent clause.

The independent or main clause in this sentence is - Their blood pressure typically does not rise with age.
The dependent clause is - where members consume little salt.

From sentence structure standpoint, this sentence is correct. However, the construction of the sentence is not correct, especially the function of modifier "in societies".

Thanks,

Payal