So the questions and answers provided by Eddie Kal in the comments pretty much answer your question, but I can see how it might not be immediately obviously how they apply in this case.
First of all, just to be clear, で can be used for expressing time/timing in certain cases. You will see this listed as a possible definition in pretty much any dictionary, including Jisho. In particular, as mentioned by the first answer and alluded to by the second answer provided by Eddie Kal, で carries a fairly strong connotation of something coming to an end or resolving at a specific time.
In regard to how that applies to this specific case, you said in the comments:
Those are regarding verbs that terminate, almost like an "until", while in this case there's no termination or deadlines, and only the day itself is significant
Arguably the process of becoming exactly 20 years old is arguably something that requires exactly 20 years and then terminates. Looking at it that way, this seems like an appropriate usage of で. If that doesn't seem intuitive to you, you can also just think of で as emphasizing the significance of the specific time.
この事故から、１０日で２０年になりました。
On the 10th, it will have been 20 years since the accident to the day.
で here is perhaps not quite as strong as English's to the day, but it's the closest thing I could come up with.
