Hard to define. The classical beginning starts with enlargement of the testis as it begins to make more male hormones.Since no-one sits around measuring these daily, gradual changes in growth, muscularity, skin eruptions etc appear and people notice.Some consider the end of bony growth as one end of puberty.Many consider the brain changes that finish about 25 as the true end.
Vcariable. The signs are well known by all and can not be predicted by age alone. He becomes more muscular, voice breaks, genitals get bigger------all obvious. The mental changes are less predictable.The boy is becoming a male adult and personality and behavior will be the result of upbringing and genetics.