Day 300... He's barely 10 months old in that world age...
What was his age pre-isekai? Kinda forgot from so long ago. Also Kanami is the same age as him, but gobbos age like what, 10x the years to human agewise...? That would still make them barely 8 years old in human age... Just found it all funny
rewatching the anime, and from what's shown there... mid-30's or so in his original world.
And the gobbo line does
mature faster, but not necessarily
age faster. It's just that if they can't evolve, their life expectancy is... low... politely put.
If they manage to get to Hobgob.. better.. From there it's flavours of Ogre, which afaik have no different life expectancy here as humans. Possibly even longer.
It's the whole 'Monsters Evolve" thing , combined with his special skill of skill absorption..
But even if he hadn't had that... Because of the evolution system for monsters, they can
rapidly evolve if they manage to get enough XP to hit that "level 100" consistently, and survive....
It's just a different version of powerlevelling, really..