Want to Be Protagonist

A 20 year old man is reincarnated into a fantasy world, he doesn't know why he can die. In that fantasy world, he expanded his knowledge and found something called an achievement. Achievement which is a supernatural power, which is only available in a grimoire. To get a grimoire the user's age requirement must be 15 years. When he reaches the age of 15, he is very excited and tries to become the main character in that fantasy world.