No doubt at all that Imam Hasan (AS) and Imam Husain (AS) are the two masters of all the youth in Paradise. This fact has been mentioned in many Hadeeths in Sunni and Shia books. The Hadeeth states also : Their father is better than them وأبوهما خير منهما. Al-Mustadrak ala al-Saheehain by al-Haakim al-Nisabori , V.3, P.182.
All believers in the Paradise will be young even if they dies in old age. There will be no old age in Paradise. The masters of all in the Paradise will be the Prophet Muhammad (SAWA) being the Best of all creatures, then Imam Ali (As) as it is mentioned in the Hadeeth, then Imam Hasan (AS) and Imam Husain (AS).