This theory isn't true.
Allah will guide all human in every age, even all creatures if he wills. According to:
He said, "Our Lord is He who gave each thing its form and then guided
"Who created and proportioned,And Who destined and [then] guided"Surat
"... And Allah guides whom He wills to a straight path."Surat
"Verily, Allah sends astray whom He wills, and guides whom He
"Then Allah misleads whom He wills and guides whom He wills."surat
"But He sends astray whom He wills and guides whom He wills."surat
On the other hand There are many verses in the Qur`an which confirm the free will of the human being, such as:
"Indeed We have guided him to the way, be he grateful or
"And say, "The truth is from your Lord, so whoever wills - let him
believe; and whoever wills - let him disbelieve...."Surat
"A group [of you] He guided, and a group deserved [to be in] error.
Indeed, they had taken the devils as allies instead of Allah while
they thought that they were guided."Surat Al-'A`rāf,verse30
So, This ability to choose is a measure of guidance from Allah (awj).human being is a free being and can choose to take the path of Divine guidance—thereby preparing in himself the grounds for further guidance—or he could choose to take the opposite direction, which leads to disbelief and oppression. So, if the human being decided to walk on to the path of disbelief and oppression, he would be naturally deprived of Divine guidance. But if he chose to traverse the path of truth, he would be blessed by that Divine guidance, without any limitation on age.