@mattdm but it isn't

it would be an additive blend, not an opacity adjustment

for double exposure, if you have a black pixel that is covered with a white pixel, you get a white pixel from the exposure, not a grey one

unless you are under-exposing to compensate

but either way

it's an additive process

now, granted, the answer was close to correct, but the OP was also correct in saying there was an error in that post. That said, the marked solution is correct, though it doesn't actually expand on what blending options need to be used

@WalidNawfalSabihi There sure are scientific analysis methods for detecting age of a photo but I do not know any specifics. Not something what a user can do on own behalf.

@AJHenderson It all depends. The combine image tool in Canon's DPP has several options for combining images: Add (B + W = W), Average (B + W = Medium Grey), Weighted (B + W = anywhere from 100%-0% B to 0%-100% W), Lighten (B + W = Light Grey), Darken (B + W = Dark Grey).

@MichaelClark right, I wasn't disagreeing that blending can do it

the answer the OP in the dupe commented specifically about the image that was just an opacity blend (ie, straight up % of each color mixing)

the OP was accurate in saying that the one answer he was looking at wasn't making an accurate simulation, but was incorrect in saying that the question wasn't a dupe or that there weren't answers there that pointed in the right direction