To compare with the girls thread I started earlier, here is what I found for the boys. The time barriers are 5 seconds plus or minus the MILESPLIT US SECOND TEAM ELITE standard of 4:25.
Year = sub 4:20 / sub 4:30
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2000 = 1 / 13
2001 = 1 / 13
2002 = 2 / 12
2003 = 3 / 15
2004 = 1 / 13
2005 = 0 / 11
2006 = 1 / 10
2007 = 1 / 11
2008 = 2 / 25
2009 = 4 / 18
2010 = 6 / 24
2011 = 4 / 23
2012 = 9 / 21