A simple yes or no based upon your knowledge will suffice in case no one can cite specific details f/ the NFL history of the rules.
I will present the results f/ your votes as support to aid in settling a dispute.

yes

i'm not sure if it was always put into those terms, but the concept hasn't changed.

Yes, but there are exeptions.... the ground cant cause a fumble because when somebody lands on the ground when being tackled they are down by contact...... but if someone is running, and nobody tuches the guy and he falls down.... hits the ground and the ball pops out... its a fumble

Ever since I can remember this has been the rule, but I'm only 22 years old so I that isn't much memory.

not always

It's not a statement, so much as a figure of speech. The reason the ground can't cause a fumble is because when the ballcarrier hits the ground, the play is over. The ground cannot cause a fumble when the player in possession is down by contact from an opponent. When a runner goes down on his own, the ground can cause a fumble, and the ball continues in play.

for as long as I have been watching football and that has been 47 or more years.