when did you purchase this snake? have you given it a week to settle in without handling? the breathing hard you mention sounds like the snake is hissing at you. that is normal i have some that do it all the time. any other weird breathing noises besides hissing should be checked out right away.

to approximate his age we would need more details but if you just purchased it from a pet store and it is a thin little guy then i would say no more than a couple months old. if hes is that size then he may be able to eat rat pinks. got any pics???