"If you aren't saved by age 18, chances are you never will be," is another way of putting it.
It makes me wonder what the underlying message is. Maybe something along the lines of "Don't bother witnessing to adults, 'cause they're not likely to listen"?
And that in turn makes me wonder who's the author of this little piece of advice. The god of this world, perhaps?
If I had to guess, I'd say that 40% of the people I know were born again when they were well into adulthood, often because of the sort of life-changing trauma that is peculiar to the over-30 crowd.
I've heard of, and known, nursing-home residents who were saved in their nineties and beyond. Thanks be to God that some have ignored conventional wisdom and shared the gospel with them, apparently not concerned that these elderly people were too old to repent and receive Christ!