I know I have to declare my newly purchased laptop from San Francisco when I land in Paris. Do I have to declare it when I am at the airport of San Francisco as well? Or can I declare it while being on the airport of San Francisco only?
There is no exit customs for ordinary travelers in the USA. The only reason I know of (and it does not apply here) is if you had a high-value object, like imported photographic equipment, on which duty is usually paid in the United States, and you want to register the serial number with Customs so that on return to the United States there is no issue about paying duty on it again.
Customs is generally about the importation of goods— barring illegal items and assessing duties on legal but taxable ones.
The U.S. has no formal exit controls, and U.S. Customs and Border Protection (CBP) does not perform any standard outbound inspection of airline passengers departing the U.S. Even if you had something to declare, you would have no place to declare it.
The airline will care insofar as they need to know if anything you have cannot legally be taken aboard an aircraft (e.g. weapons, fire starters), but a laptop is not much of a concern.
Yes, it is illegal to take many things out of the country— say, eagle feathers, or some cryptographic technology, or things controlled by international treaty like counterfeit products or drugs. But the list of those items is not large enough, nor the likelihood of a random passenger carrying them, to justify the resources needed for passenger-level inspections from the U.S. Instead, the authorities will X-ray or hand-inspect checked luggage, and only interact with you if there is a suspicion of wrongdoing.
Besides, even if you bought the laptop new in San Francisco, it was probably imported from Taiwan or South Korea in the first place.