I'll never understand people claiming Rudeus has a mental age of 40+ simply because he has memories of his past life. What actually determines someone's mental maturity? Isn't it influenced by brain development and life experiences? So what exactly makes Rudeus mentally 40+?
First of all, he was reincarnated into a new body with a developing brain. Secondly, he spent years of his previous life as a shut-in, isolating himself from society and remaining confined to the same environment. This contributed to his psychological and social stagnation, making him emotionally and psychologically stunted
You can't simply add together all the years he's lived and call that his mental age. Even in real life, people of the same age can have vastly different levels of psychological maturity let alone someone who's been reincarnated into a completely new body with a developing brain