The romance genre in Japanese anime is primarily skewed to tell stories of the blossoming love between teenagers. While these stories can be genuinely great romantic tales, adult viewers would like to see the love between older, more mature characters whom those viewers can relate to more deeply. Even if anime often caters to tweens and teenagers, plenty of adult fans are tuning in too, and romance fans in their 20s and 30s would much rather watch adult romance anime than yet another "two high school students fall in love" series.
Source：https://www.cbr.com/romance-anime-for-adults/
