When he reached full age, and was firmly established (in life), We bestowed on him wisdom and knowledge: for thus do We reward those who do good.
Full age may be taken to be mature youth, say
between 18 and 30 years of age. By that time a person
is fully established in life; his physical build is completed,
and his mental and moral habits are formed.
In this case, as Moses was good at heart, true and loyal
to his people, and obedient and just to those among
whom he lived, he was granted wisdom and knowledge
from on high, to be used for the times of conflict which
were coming for him.
His internal development being complete, he now goes
out into the outer world, where he is again tried and
proved, until he gets his divine commission.
Yusuf Ali, Abdullah. Translation and Commentary of the Quran. (surah 28 verse 14). Quran Arabic with English Translation & Commentary (Tafsir) by Abdullah Yusuf Ali, Free Download (quran4u.com)