In Act 3 of the Manga Mamoru is confirmed to be a 16-year-old high school student whereas in the anime he is a college student. Whilst this might mean he is 18, according to the Sailor Moon R Movie Memorial Album, Mamoru would have been 17 at the very start of the anime and turn 18 later on. Fans often point out/complain about Mamoru being aged up in the anime and have struggled as to why for years, often presuming bad faith on the part of the anime staff.
There has never been a confirmed reason to my knowledge, but I have come across something that seems to make a lot of sense to my mind.
Ultimately, this hinges upon the production realities of the Sailor Moon anime, which were atypical for how most manga-to-anime adaptations work. Whilst an episode of an anime can be produced from scratch in about two months, usually the manga the anime is adapting is many months, maybe even YEARS ahead of the anime. That wasn’t the case for Sailor Moon.
The Sailor Moon anime began airing on a weekly basis on 7th March 1992. But the first chapter of the manga (which episode 1 adapted) was released on 28th December 1991, the manga then being released monthly thereafter. This accounts for why there were SO MANY filler episodes in Sailor Moon, but it also speaks to the insane deadlines the anime were operating under.
Whilst I couldn’t find the original sources for this, this article by Tuxedo Unmasked explains that Naoko Takeuchi was involved in the production of the anime, at least somewhat early on in the first season. How involved is a little unclear and it is likely that as the manga pushed forwards and Sailor Moon became more popular she became busier.*
It is extremely likely that Takeuchi was giving the anime staff notes before the first episode even aired, the better to give them stuff to work with to generate filler episodes. It just so happens we have access to some of Takeuchi’s early notes for Sailor Moon in the form of the Materials Collection:
Note how Shingo’s note lists ‘Mika’ as one of his weaknesses. In the manga, no character by the name of Mika ever appears, at least not affiliated with Shingo in any way. However, a classmate of Shingo named Mika does appear in episodes 5 and 18 of the anime and he does in fact have a crush on her.
Whilst specific release dates for the manga are not confirmed (merely publication dates which are not the same thing), if we presume Sailor Moon kept to a regular monthly release schedule the character of Reika Nishimura would have appeared in late October 1992, specifically in Act 11 of the manga. This would have been around the same time that she debuted in the anime, specifically in episode 29 on 24th October 1992. Regardless of which came first, obviously the anime staff wouldn’t have made an episode from scratch in such a short space of time. They clearly knew about her ahead of time.
So, to some degree Naoko Takeuchi was feeding the anime staff ideas and notes on her story. Here is the problem though…her story was an evolving process. Case in point, Takeuchi originally intended Ami/Sailor Mercury to be a cyborg and there is even a hint of this in Act 2 of the manga
Clearly she changed her mind about that, and this wasn’t an isolated incident either.
In her linear notes for Act 3, Naoko Takeuchi herself states that she pitched three different ways Jadeite could become involved with the Hikawa Shrine in the anime. But when it came time to make Chapter 3 of the manga, she changed her mind last minute so that Jadeite was never involved with the shrine in the manga, but was nevertheless working there in the anime.
The Materials Collection is perhaps the ultimate example of how Takuechi revised the story she was telling as she went along.
Remember how Sailor Mars occasionally didn’t wear gloves and shot lasers from her fingernails?
With all this in mind, lets take a looksee at Mamoru’s pages from the Materials Collection:
Oh me oh my. As you can see, early on, Naoko Takuechi herself intended Mamoru to be roughly the same age as he was in the anime.
In other words, what seems to have happened was that the anime didn’t not age Mamoru up. They simply depicted him in line with the original intentions of Naoko Takeuchi. And then at some point before or during the production of Act 3 of the manga, she simply changed her mind.
*This in turn might account in part for why the anime gradually deviated from the manga more and more, to the point of not bothering to adapt anything after chapter 4.
P.S. Check out how Mamoru is depicted on the next page of the Materials Collections:
Yep…he is holding a red rose.
Could it be the other major difference between Manga and Anime Mamoru was also thanks to Takeuchi’s early ideas?