It does make sense. At least, for the LT. Apparently he\she survived OPERATION: TORPEDO, which was when the UNSC sent 300 Spartan-III's to Pegasi Delta, in 2545. The Battle of Reach occurs in 2552, so say the LT was 15 during the operation, and did infact survive, that would make him 22 at the time of the Battle of Reach. I can't say the same about the other four because we don't know anything about their origins.