There are about 250 million adults in the US.
I would take that to mean that if you're a US adult, with any sort of credit history, you're affected. The affected data for the larger 143 million was: "Most of the consumer information accessed includes names, Social Security numbers, birth dates, addresses, and in some instances, driver’s license numbers."
There's also a smaller set that had even more data exposed:
"In addition, credit card numbers for approximately 209,000 consumers and certain dispute documents, which included personal identifying information, for approximately 182,000 consumers were accessed."
However Equifax offers other services such as Background Checks which more americans are passing off their SSN to, generally unintentionally (its mandatory for work).
This leads me to believe even young people are affected if theyve taken an internship or anywhere (AT ALL) that uses Equifax.
Its scary how little information the media is providing on this. Equifax does not provide an FAQ over what conditions you may be affected. I don't have a line of credit, and I have never used their services personally, HOWEVER, if a prior employer used them through a background check, or if they used a 3rd party who sends my data to equifax without me knowing, I'm pwned and didn't even know it.