Actually, none of the character's are middle aged. People that are significantly younger than someone's age (for instance prob in your case, younger than these character's, or older in fact) may feel that way however. Middle aged is actually aged....well in the middle of one's life. Between the ages of 45-65. This depends of course, on one's familial life expectancy genetics. But ya, middle aged doesn't even start until 40-45 for most individuals.
Now side note: these are VNs. They come in all sorts of perspectives, and usually not your own. If your looking to play more BL games in your perspective, try searching for dating sims. Some great examples are: "Signed and Sealed with a Kiss", "Seiyuu Danshi", "Dead Dating", "Banana Ranch(same creator as Seiyuu Danshi, but this game is WIP. Won't be out until Dec 2022)", "Uncle Neighbor *on steam*", "Dream Daddy", or even "Coming Out on Top." These are some great stories and art content you might miss out on, if all you worry about is whether the character's are between 18-20yrs old😓😭😭. I urge you to def try them out tho and see how you feel! I've gotten past that myself as I've gotten older and realized most of VNs or Otome games I love tend to favor the younger adults.
But I do get what you mean about it being nice to have character's near your age🙌😆. I'm in the middle of my 20s, so I'm actually quite excited to finally play a game that doesn't revolve around 18yr Olds😂(cough, cough, ignoring the fact that the Protagonist is 18 👀 , "every otome game in existence" 🤣) Also, VNs come in multiple perspectives as well as with their own subgenres (like playing a VN with rpg like features) ex: Winter Wolves games tends to be like this.
So, if u are looking for more of blank slate character's, try the game's I listed up above (*If you can get past most of the character's being older than 20yrs old tho😅, or even translations for Eng being a bit off due to some still WIPs* You might actually really come to love those games. Or if not, just continue searching specifically for VNs that are mostly Dating Sims. Just keep in mind, you can find games with character's your age, but that will severely limit the amount of great-ness you can enjoy😍.