Praise be to Allah, and peace and blessings be upon the Messenger of Allah. To proceed:
Allah Almighty says, describing the bliss of the people of Paradise:
“There will circulate among them young boys made eternal. When you see them, you would think them scattered pearls.” [Al-Insan 76:19]
And He سبحانه وتعالى says:
“There will circulate among them everlasting youths.” [Al-Waqi‘ah 56:17]
The meaning of the everlasting youths is: young servants created in Paradise who circulate among its people to serve them. They remain in one condition; they do not grow old, nor do their conditions change.
The scholars differed concerning their exact nature.
It was said that they are the Muslim children who died when they were young. It was also said that they are the children of polytheists. However, there is no authentic evidence establishing these opinions. Shaykh al-Islam Ibn Taymiyyah رحمه الله stated that the view that they are Muslim children has no basis. It was also transmitted from Ibn al-Qayyim that the hadith reported concerning this matter is not authentic.
The view preferred by Shaykh al-Islam Ibn Taymiyyah, and regarded as more likely by Ibn al-Qayyim, is that the everlasting youths are a creation among the creations of Paradise. Allah created them to serve its inhabitants, and they are not the children of people of this world who died when they were young.
As for the children of believers who died before reaching puberty, if they enter Paradise, they will be among its inhabitants, and their creation will be completed in the most perfect form. They are not the youths whom Allah appointed to serve the people of Paradise.
Therefore, the stronger view is that the everlasting youths are not the Muslim children who died young. Rather, they are a creation whom Allah prepared in Paradise to serve its inhabitants.
And Allah knows best.