Those are termed as the ‘hidden years’ of Jesus, since the Bible does not relate anything about Jesus from His birth until His twelfth year, and from there until His appearance (as a thirty year-old) at the Jordan Area.
We can presume that, like Joseph, He worked as a ‘tekton’ (a carpenter/workman), perhaps doing anything from making tables to working on buildings.
Just as mentioned above, there are many Apocryphal Gospels which show Jesus, first as some kind of ‘holy terror’ and an ‘infant prodigy’ (such as pushing off His playmates from a roof and raising them up again, striking blind and lame the parents of playmates who complain to Joseph and Mary, correcting and explaining the letters of the alphabet in depth to a teacher who offered to teach Him His letters) who eventually, as He grows up, use His Power for more benign purposes (such as healing James from a snake bite and a man or boy who accidentally chopped off his foot, lengthening a piece of wood by stretching it or carrying water using His cloak since He broke His pitcher).
As for why Jesus started when He was already 30, this was because back then, a man below his thirties would still be considered ‘young’ and a ‘juvenile’. I guess He decided to start at the age wherein He would be considered as living in the prime of His life, i.e. a full-fledged adult.