How many people under 30 voted in the 2008 Presidential election?

Well voter turnout was about 61.0% of those assumed eligible to vote.

Assumed 215,000,000 so about 84,000,000 people who most likely could vote, did not.

As a side note. A 2/3 majority requires 67% of those eligible to vote to participate.

So if in order to have a president at all, the requirement was changed too 2/3 participation necessary, then the last time any one would have been elected to office would have been around the late 1800's.