Just a quick, trivial sidenote: we do actually grow new brain cells. The losing brain cells theory (except in the case of physiological trauma, of course), which never made a whole lot of sense to me anyway, has in recent years been disproven.

So don't worry, your brain will be alive and kicking for many years to come!

P.S. Here's one article I found with a quick search. It's not the one I read, but it has similar conclusions.
http://www.princeton.edu/pr/news/99/q4/1014-brain.htm