«حَتَّىٰ إِذَا بَلَغَ أَشُدَّهُ وَبَلَغَ أَرْبَعِينَ سَنَةً»
“Until, when he reaches maturity and reaches forty years.” (al-Aḥqāf 46:15, excerpt)
“Reaching full strength” names a general stage of maturity; “reaching forty years” provides a specific age within the description. The coordination links an overall quality to a definite point in life.
In explanatory coordination the second element can illuminate the first rather than merely adding an unrelated fact. The verse should not, however, be converted into a universal biological claim that every human capacity attains its maximum at precisely forty. The passage concerns maturity, gratitude and duties toward parents.