They'll stop nagging when you're doctor/pharmacist/dentist/engineer/lawyer. Hell, I still get nagged after all these years but being passed my teenage angst, I appreciate their nagging more and more.
It means they're still healthy and strong enough to worry about me. I'll fucking hate the day they stop nagging because I know what will be coming after.
Most often, once your parents stop, you'll wish to hear them nag at you again.