Beware of the causal claims. Sure, kids and adults rely, to differing extents, on different regions. That's true of almost every domain. Real world pressures - active and passive learning - are influencing the activity and connectivity patterns throughout life. For instance, older individuals compensate for decreased brain functioning by shifting activation to other, supporting regions. Folks are reluctant to call that "reorganization". It's a change in activation (as this study shows) without a clear change in structure (which this study doesn't appear to show).

Here's a good review of recent number work: http://www.wjh.harvard.edu/~lds/pdfs/feigenson2004.pdf

Another involving specifically the parietal cortex: http://www.unicog.org/publications/HubbardPiazzaPinelDehaene...

Andreas Nieder and Earl Miller have also shown some really cool effects for how number relies on specific response properties of individual neurons in non-human primates:

http://web.mit.edu/ekmiller/Public/www/miller/Publications/N...

http://web.mit.edu/ekmiller/Public/www/miller/Publications/N...