Is the statement, made in ignorance and recklessness during adolescence, "I am guaranteed to live until tomorrow," considered disbelief that nullifies subsequent good deeds such as Hajj and acts of obedience?

If the statement was uttered mistakenly, unintentionally misrepresenting the word's true meaning, then there is nothing binding on the questioner. However, if he said it in jest, it is feared that it might be an act of disbelief (kufr) because only Allah knows the unseen. Jesting in such a matter does not prevent the one who utters it from falling into disbelief. Whoever speaks a word of disbelief, whether in jest or play, is considered a disbeliever by all.

Therefore, repentance for this statement is obligatory. If he repents and regrets, he is hoped to be forgiven, and his Hajj and all his other acts of obedience remain intact, as the preponderant opinion is that deeds are not nullified merely by apostasy, but rather by dying upon it.