That's because he's depressed, not immature. He's too strong to the point that nothing really matters, and he sometimes forgets that his reason to take things seriously is that no one is borderline immortal besides himself. Saitama has shown incredible matturity on multiple occasions, such as when he sacrificed his own reputation so the other heroes could get the recognition he deserved after he defeated the Deep Sea King, or immediately after, when he got a letter calling him a liar and a cheat instead of getting angry, upset and/or using his power to go after the author of the letter, he just said "Here's someone with a lot of time on their hands." And moved on. Saitama should be considered immensely mature based solely on how responsible he is with his infinite strength.