There's really no set age that an adult or adults start losing their teeth.
However some adults start losing some of their teeth by age 44 to 50 years old and some adults may not lose their teeth until they are 80 to 90 years old.
But then again some adults never lose their teeth and have their teeth until death.
Then of course after the person dies and is buried then the teeth will eventually rot out and they will lose them but the adult will no longer need their teeth when they die.
As long as you keep your teeth clean and healthy and get dental checkups often you may never lose your adult teeth once they come in.
But if you do not take care of your teeth or see a dentist often to fix any potential gum disease or tooth decay before it gets too severe.