For 18% levoscoliosis and 10% dextroscoliosis, is a brace necessary?

What age??: Those numbers are right on the border of where more aggressive treatment such as a brace may be considered, but that decision has to take into account your age and further growth potential. If you are well past puberty/adult, bracing may not do much for you. If still growing and a teen or pre-teen, may be very useful to prevent further progression of the curves. Also depends on area of spine.

Depends on several : Factors but first, a levoscoliosis is not the usual presentation & may warrant an MRI for further investigation as to cause as it is not the secondary curve as it is the larger of the two. Otherwise, age & skeletal growth left, family history, curve size as well as any sign of progression would determine need for brace or observation. This is a mild curve but if skeletally immature, observe.