A few days ago I posted which subreddits each age group is most overrepresented in (https://www.reddit.com/r/dataisbeautiful/comments/1xf6gt3/). The pushback I got most: people on Reddit lie about their age, self-reported data can't be verified, it's just noise.
Fair point. So instead of arguing, I checked.
Chart 1: do people age on schedule? If people made their ages up, there'd be no pattern over time. So I took everyone who stated their age in two posts or comments at least a year apart, 2.3 million people, and checked whether their age went up by the time that actually passed. 49% aged exactly on schedule and 68% were within a year. The outlined bars are the same people with their second age swapped for a random stranger's, which is what noise looks like: 3.5% and 7%. Gender too: 5.4M people stated it twice and 96% said the same thing both times.
First, what isn't a lie. Plenty of numbers look like ages and aren't: "I'm 99 farming" is a RuneScape skill level, "91F" is an army job code, "Female, 26, nurse" is someone guessing about another person. I filter that out wherever I can spot it, and nobody's age comes from one comment anyway: it's the middle of everything that person has said, so a stray number gets outvoted.
Chart 2: so where do everyday Redditors lie? I flagged every age claim 10+ years off from what that same person usually says, then read 20 random flagged posts in each of the 100 biggest everyday subreddits. Three AI reviewers per post, and a lie only counts when all three agree. Result: about 1 in 270 age claims is a lie. Even the "worst" one, r/indiasocial, is around 2%. When people do it, they mostly add years: 38 in the post, 26 everywhere else. Think a r/tifu story told by a 50-year-old from an account that's 27 in every other post, or 30-somethings saying they're 15 in r/teenagersnew.
Chart 3: where it spikes. Dating subs are even more honest than average (0.15%). Lying only takes off in a few adult subs: 8.4% of age claims in cheating and cuckold subs, 3.5% in "mature women" ones. 91% of those posts are written as a woman and 65% come with a Snapchat handle. Promo accounts posting as a 45-year-old woman, who are 28 in their other posts.
So: most people just say how old they are.
Full write-up with five more checks, where this method is weaker, and the CSVs: https://kissmypost.com/research/reddit-self-reported-age-gender-accuracy
Source and tools in the first comment.