Cost could well be a factor in the decisions but, depending on numbers, perhaps not a large proportion of the present cost of airport security. This website estimates costs as:
While the TSA usually cites scanner costs ranging from $140,000 to $180,000 apiece, this is only the scanner cost and does not include associated airport modifications and installation expenses.
http://tsanewsblog.com/1847/news/time-for-a-rational-perspective-on-airport-scanners/
Well if you find the real reason you let me know OK
But if the they make a rule to not search kids under 5 years old where do you think the next bomb or gun will be hidden.