It also took the "train your magic limits as a child" concept from Mushoku Tensei. Still, this is all set up and the plot is likely starting after the introduction arc. I'll reserve my judgment.
That part makes sense in some ways actually. Human body isn't really mature or adult till around age 25 and the age of 20 in this story is close enough to that. The elven age of 300 doesn't really scale with that though, unless thinking that human max age is less than 100.
In any case, there are all sort of health issues that start cropping up after age 25/30 and it can be easily said that ones peak is around the age 25 or so. Certainly physical strength can be trained for a very long time but body maturity is probably fairly realistic in the above age manner and magic capacity training could work with that fairly well if assuming it's part of the growth of the body.
The above age range is also why I would never write a story where 15 year old teenagers get some superpowers somewhere. Perhaps it works regarding whoever the audience is, probably children somewhere, but its really no different from the insanity of giving a nuclear device for a teenager and then expecting them not to denote it due to hormones, volatility and what not
All in all this story does have some promise at least, but as others mentioned the pacing was very fast regarding the beginning. But perhaps the author wanted to get that out of the way so that they could focus in everything else that comes after that.