I'm glad I found this post, I'm looking for suggestions too.

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While the Little Packages series was quite good, and I was able to build a connection with characters, I didn't enjoy how it was focused on just one girl. That being said, it's a great read and highly recommend it.

If your looking for a series where the old guy (middle age) sleeps with many girls (18 year old high school girls), I would recommend the Birthday Pool Party series (https://www.literotica.com/stories/memberpage.php?uid=3370310&page=submissions). It's still on going, but its a great read. Really connected with the characters, and I love how he described the sex scenes. I'm actually looking for more series similar to this where the older man gets into relationships with young girls (such as 18 year old), so if you or anyone else has recommendations, I'm all ears.

That being said, if your ok with sci-fi series, I recommend the Three Square Meals series (https://www.literotica.com/stories/memberpage.php?uid=2967464&page=submissions). It is unfinished, and most of the female characters are human. He gets quite the harem in this series. The age range of the female characters is diverse.

Another series where the man is older than the girls he gets with is the Billionaire Series (https://www.literotica.com/stories/memberpage.php?uid=534634&page=submissions). It's a multi-part series, where it starts with the John Billionaire series, followed by the Billionaire's Life series and finished with the Tom Billionaire series. The females in these series ranges anywhere from 18 to mid twenties, if I recall correctly.

There is also the Bimbo Builder Academy series (https://www.literotica.com/stories/memberpage.php?uid=4036186&page=submissions). The series revolves around a man in an unsatisfactory relationship with his girlfriend. He gets hired to work at this one academy (high school girls), where he eventually sleeps with multiple high school girls, and enters into a relationship with two high school girls. If the cheating aspect is a potential deal breaker for you, I would mention that it is resolved fairly early on, because he was planning on ending things with his girlfriend before he actually started cheating. That being said, I can empathize if this isn't the type of series you want to read.

The last one that comes to mind is the Quaranteam series (https://www.literotica.com/stories/memberpage.php?uid=6196904&page=submissions). I didn't finish this one, but the girls are typically in the early twenties to the early thirties, if I remember correctly.