Yuukyuu no Gusha Asley no, Kenja no Susume  

The principle of a philsopher by eternal fool "Asley", 悠久の愚者アズリーの、賢者のすゝめ

In his youth, Asley was expelled from the Magic Academy and faced ridicule from others. However, by chance, he stumbled upon a god medicine that granted him an immortal body. Driven by revenge against those who mocked him, Asley dedicated himself to researching magic and magical artifacts unlike those created in the academy. Unbeknownst to him, 5000 years had passed before he realized his transformation into an ancient magic user. Now, with his familiar Pochi by his side, Asley embarks on a journey to explore the world after being away for half a millennium. Along the way, unexpected events occur and he finds himself living at the Magic Academy once more. How will this 5000-year-old with a youthful appearance navigate his second life?