Why did my back start aching? it never used to, i am turning 40 soon though.

Too many causes: It's hard to give you a definite answer without a lot more info. Having said that, many people lose flex ability and agility with time and actions that were once well tolerated will cause an overuse injury. Treatment of overuse is rest and avoidance of re-injury. Try to determine what started the pain and avoid that at least for 2 weeks. If you are continuing the activity, resume it gradually.