Do martyrs become more youthful, and is there any evidence for this in Islamic law?

There is no explicit text stating that a martyr grows younger. However, numerous virtues have been affirmed for him, including being married to seventy-two wives from among the Hur al-Ain, as mentioned in the Hadith that every person in Paradise will have two wives from among the Hur al-Ain, and the martyr is among the believers most honored in Paradise.