Sitters of the right age for:-
Uniform [Military or job related]
Marriage [Not always obvious]
Seventeen [Girls of that age]
Twenty-one [Voting age at this time]
First Job [Age/Clothing]
Birth [Always a baby]
Holiday [clothing/backdrop/seaside photographer]
Near Christmas [photo to send to relatives]
Group of Youngsters [shared the cost]
Pair of Friends [split the cost]
Whole Family [Mum Dad & kids]
Sent the money [by a relative who wanted a picture]
Special Offer [Photographers often had special rates]
The list is endless and unless something is written on the print or there is a clue in the picture you can only guess.
The silent sitter of every photograph knew the reason it was taken but they are not speaking.