i don't think i've discussed this on tumblr before outside of a few offhand mentions, but a while back i put together scraps pointing to the fact that all of dennis' erotic memoirs were written while he was in high school, primarily backed up by the fact that the two additional memoirs posted on the official paddy's pub blog as well as the other two that were adapted into a dvd extra, which either directly mention this
(which makes him 16-17) and explicitly ends with him allegedly having sex with multiple adult women and a tiger, or implies it – dennis saying he drank 12 mugs of "mead" then later saying it was "brandywine", which is very much not mead while also specifically mentioning a non-alcoholic beer sign. the NJ location of medieval times opened in '90; he mentions going with A Companion (which was probably a date he drove there, not mac or charlie,) so I'd place this around the same time (16-17), after he got his license, but before he was legal drinking age.
when he first reads from his memoirs in 4x09, he narrates one that states "she was much, much older than me", and in the dvd extras he also recounts sleeping with a "matronly" japanese woman who also ended up being a "good deal older" than he had thought she was, and notably says, "nestled in her ample womb, i became a man" which has. certain implications.* in the second dvd extra he also says "i'm pretty sure i was her first" re: the random woman who he's picked up, which i'd say was originally written and intended to have taken place when he was significantly younger. obviously the reenactment has them both as 30-something yr old adults, but smut film also has him reprising the role of his 14 year old self, so it's not without precedent. he also starts off saying this is from chapter two, and the prior story was from chapter one, implying both are extremely early on in the memoirs
there's also the fact that charlie had stolen the memoirs for an unspecified length of time, long enough for dennis to comment on how long it'd been since he'd seen them. we can assume he likely stopped writing the memoirs entirely once he got his recording setup, which would've been around the time he moved in with mac somewhere around '98 when they bought the bar, putting him around 21/22. but the earliest memoir we know of dates back to 1991, when he was 14, then another confirmed to be in 1993/1994. additionally we only see him adding to the book when he's institutionalized (which is implied to have been some sort of hallucination, so it's arguable that even happened) and in the dvd extras (with a quill but no ink, so i don't know how that would've worked).
tldr; dennis sleeping with older women while underage was a pattern, possibly before klinsky, definitely after, and all but one of his memoirs seem to explicitly detail this abuse.
*i'm fairly certain this was his account of losing his virginity, and i believe this predates klinsky. if this truly is "chapter one" of the book, then it's more or less confirmed, but this memoir is also an interesting outlier because dennis actually has an emotional reaction after sex and goes back on his ~pinkie promise~ about not falling in love with her. just like with klinsky, she's portrayed as alluring (something that makes up for her age in dennis' eyes, also similar to the memoir snippet we hear him read in 4x09) and there's a heavy focus on her being motherly, but she is very much depicted as predatory as well (particularly via editing choices – clearly depicting her as a cougar – but also dennis' narration), with her seducing him, then Separating Entirely.
