Here Mature does not means by the age. It means how they understand situation and care about others and what their maturity level.
Asta from Black Clover series. He is not too dumb but he can understand situation properly.
Natsu from Fairy Tail series. He is a balanced main charecter. He is not too dumb and he understand situation perfectly.
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Senku from Dr. stone series. He is smart, intelligent and he can understand properly situation and react according to that.
At first luffy looks like dumb pirates like his family. He can do everything for them. he lent his strength to Luffy for rescuing Ace.
Koro sensei from assassination's classroom. He understand each situation perfectly and his sacrifises are insane.
Gon from Hunter X Hunter. He is one of the most matured because he understand his dad's situation even he is a child. His understanding on each situation is incredible.
L from Death Note Series. He is one of the smartest charecter of all time. His judgement and analytical skill shows the maturity level of him
Tanjiro from Demon Slayer Series. He is kind and matured even he can understand the situation of Demons who are his emenies.
Naruto is a bit reckless but he is more mature than other anime charecters. How is develope himself to gain respect after that much ignoring is outstanding.