When I was 27 I dated a 19 year old for like a month and there was just nothing to work with. This is 4 years ago now, but I remember she had literally no clue on how to pick up on my sense of humor and that I had to explain everything to her. I distinctly remember towards the end of spending time with her she made some comment about how I make her feel stupid and that I didn't care about what she had to say, which was true because she was literally clueless. Now, this girl was literally only just barely 19, so all of what I'm mentioning on here would be totally normal for someone of her age.
Based upon my observations I've actually noticed, at least in my area, that there are women that are into their mid 20s who still operate on a high school level, at least in regards to their social interactions with the world, dating, etc. I'd say at my age 31, that I'd honestly set my bar to as low as 25 and would be very selective with going younger than that and would try to filter out women who are socially immature.