Can mild scoliosis become worse with age?

Yes it can: sure it can. The more the Cobb angle, the higher the risk/rate of progression over time.

Scoliosis : No. The cut off seems to be a Cobb angle of 40 degrees. Greater than 40 degrees will likely have progression thoracic scoliosis in later in adult life. Lumbar scoliosis is a different situation which is usually degenerative disc disease and will have a different natural history of progression.

Yes: Progression depends on size of curve and age. To assess your chances of progression go to : http://espine.com/scoliosis/