semen production starts in most males at 12 years (average) - but will depend on health, diet, genetic factors and many others. the youngest confirmed case that i ever read about was age 8 (with no other medical conditions to cause it)
There are a lot of physical and psychological changes to cope with for a boy during Puberty. Many things can seem confusing for him, and the messages he is receiving from his brain will also confuse...
Boys have testosterone circulating through their bodies from before they are born, but the amount really starts to pick up around the age of 10-11, and this is what jump starts them into puberty.