Do our characters (my character and my recruit's character) have to be same level in order to receive triple experience?
No, but you must be within 4 levels of one another in order for both characters to receive triple experience. If this level requirement is not met, only the lower level character will receive the bonus.
For example: Should your character be 5 levels higher than your recruit, only your recruit's character will be eligible for triple experience.
http://us.blizzard.com/support/artic...rticleId=25716
This will not catch your lower level guy up. Just help him get back to the 4 level difference.