If your young you shouldnt have back ache.... Maybe you have an STD.....Clamydia (sp?) is known to make your lower back sore....You might not have any other symptons...
If I shoveled snow at all, I would not be able to walk for a week. Two things that might help your young back, buy a shovel that is used to push the snow instead of lifting and tossing and do situps on a regular basis to strengthen your lower back.
My back hurts but thats because I use a dirt shovel, our driveway is long and I am lazy. I just pour water on the driveway to melt the snow most of the time.