In this verse Allah (swt) uses the active form of the verb (Passing), whereas in a previous verse, Allah (swt) uses the same verb in passive form:

 وَيُطَافُ عَلَيْهِم بِآنِيَةٍ مِّن فِضَّةٍ وَأَكْوَابٍ كَانَتْ قَوَارِيرَا 

(Passing around them are vessels of silver, and cups of crystal) (Al-Insan: 15)

In the previous verse, Allah (swt) tells us about the vessels, the jugs and the cups that are passed to the believers. But in this verse of discussion, Allah (swt) tells us about those who serve the believers with the cups and jugs of drinks.

(… وَيَطُوفُ عَلَيْهِمْ وِلْدَانٌ مُخَلَّدُونَ) (Passing among them are eternalized youths. …): They are eternalized youths, never age or grow old, rather they remain in the same state without any change in their age. Allah (swt) describes those youths of Paradise as (eternalized) despite eternalization is not an attribute specially for the youths; rather, all the dwellers of Paradise are eternalized, they neither go grey nor age.

  (  إِذَا رَأَيْتَهُمْ حَسِبْتَهُمْ لُؤْلُؤًا مَنْثُورًا… ) (…If you see them, you would think them sprinkled pearls): This is one of the wonders of the Holy Quran. The scattered pearls are different from the well-protected pearls, Allah (swt) says:

وَيَطُوفُ عَلَيْهِمْ غِلْمَانٌ لَّهُمْ كَأَنَّهُمْ لُؤْلُؤٌ مَّكْنُونٌ 

(There will circulate among them [servant] boys [especially] for them, as if they were pearls well-protected.) (At-Tur: 24)

The hidden pearls are those well protected and preserved in their places. In another verse, Allah (swt) describes the (fair women) in Paradise as well protected pearls also:

 وَحُورٌ عِينٌ (22) كَأَمْثَالِ اللُّؤْلُؤِ الْمَكْنُونِ

(And [for them are] fair women with large, [beautiful] eyes, (22) The likenesses of pearls well-protected,) (Al-Waqi’a: 22-23)

They are as well-protected pearls in the seashell, so that have not been touched. Ibn Jubair said: “Nothing but the precious and very expensive things are preserved so that they could never be touched.”

(  إِذَا رَأَيْتَهُمْ حَسِبْتَهُمْ لُؤْلُؤًا مَنْثُورًا… ) (…. If you see them, you would think them sprinkled pearls): It is as if Allah (swt) wants us to see the view from outside the Paradise. Why does Allah (swt) mention the (pearls) specifically? This is because the pearls are clear and precious, just as the rubies and corals in their shining, brightness and preciousness. Since the pearls are white, Allah (swt) describes the boys and youths as pearls for their white and clear faces.