I read (from Nature Volume507, S4–S7 (06 March 2014) :
For reasons that are poorly understood, BCG protects only infants; it is ineffective in older children and adults. Its efficacy also depends on latitude. BCG works better, and protects people at later ages, farther from the equator. Both effects may result from interfering exposure to noninfectious mycobacteria that are closely related to both BCG and TB. These bacteria are more abundant closer to the equator, and while infants are unlikely to have encountered them, children and adults have more exposure over time.
I am unable to understand the relation between exposure to noninfectious mycobacteria and lessening of efficacy of BCG vacccine. What exactly is the "interference" caused by the noninfectious mycobacteria ?