I have no real age limit. Graphics can make a game feel older, but i've played enough indie games to desensitize myself to that.
I read the pre-2000 rance series games translated to english. Kichikou rance was a fun experience. The others not as much but still playable.
I've always thought it a bit dangerous to try to "learn" from fiction just because what happens in a story can be so different from real life.
You might have to unlearn it again just because the author was clueless/dishonest/just writing a beautiful story without trying to be realistic/etc.
How about https://vndb.org/v15473
Only 2 out of 5 girls on the cover have routes. You can pick side characters outside the "harem" sitting around in your house.
There's also market saturation to consider. The first sakura game sold a lot more than later ones did (according to http://steamspy.com). Pretty sure it's the same for nekopara.
The danger with episodic game releases is that your sales might collapse completely for part 2 when you don't have that new and interesting factor giving you publicity.
And of course if someone still has your first game on their ever expanding backlog they might not buy the second until they do manage to finish it.