He must be a true gentic freak.
He has good genetics, is this going to turn in to a debate about whether some are more genetically gifted than others, Mozart was writing symphony's at 5 years old, it is what it is.
His strength was never anything crazy and he always used to train the same muscle groups three times a week, he hasn't gained in years, probably stopped around 40 i would say, he stops training he gets fatter and loses muscle but it comes back quickly.
Which is why i would always advocate building a natural base, it has far more permanence.
Nobody knows what the natural limits are because everybody hops on gear nowadays at about 15.