They can get that information from marketing agencies.
Where do the marketing agencies get the information?
From insurance companies.
Examples?
AARP - American Association for Retired Persons :
These people have been sending me letters 4 times a year since I turned 50.
I have no idea where they got any information about me.
AAA - American Automobile Association
I have been a member for many decades.
If I gave them my age, it was done so long ago I don't remember doing it.
This "organization" is also affiliated with a life insurance and a vehicle insurance business.
And both of those businesses know my age.