"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.
Just yesterday, I heard about a nursing-home resident who was saved at age 102. Thanks be to God that someone ignored conventional wisdom and shared the gospel with her, apparently not concerned that she was too old to become a believer!