All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
We do not know exactly which Hadeeth you referred to in the question and mentioned that it is in Saheeh Al-Bukhari and Saheeh Muslim and other books of Hadeeth. What we have come across is the Hadeeth reported by At-Tirmithi and which was narrated by Usaamah Ibn Zayd who said: “I knocked on the door of the Prophet for a need and he said: “These (Al-Hasan and Al-Husayn ) are my sons and my daughter's sons. O Allaah, I love them, so love them, and love whoever loves them."
Al-Mubaarak Poori said: “i.e. these are my sons’, by decree [because one’s grandson is considered as one’s son], and these are ‘my daughter’s sons’, in reality [as she is their mother].”
As regards the other Hadeeth which was reported by Ahmad, At-Tirmithi, Ibn Maajah and An-Nasaa’i, it was narrated by Abu Sa’eed Al-Khudri who said: “The Prophet said: ''Al-Hasan and Al-Husayn are the masters of the youth of Paradise.” Shaykh Al-Albaani classified it as Saheeh [sound].
The meaning of this Hadeeth has been clarified by many statements which were summarized by Al-Mubaarak Poori who said: “It means they are the best among those who died as young in the Cause of Allaah from the people of Paradise. He did not mean that they will die at the age of youth as they died in their middle age [elderly], but what is meant is the chivalry of the youth, in the same way that we call someone who is old ‘a young man’ referring to his strength and chivalry. Or that he meant that they are the masters of the people of Paradise with the exception of the Prophets and the rightly guided Caliphs, because all the people of Paradise are of the same age, which is young age [33 years old], and there are no elderly or middle aged people amongst them."
Moreover, At-Teebi said: “It could mean that they are now the masters of the youth who are among the people of Paradise from among the youth of this (their) era.”
Allaah Knows best.