If my understanding is correct, subreddits that host NSFW material (or at least, a substantial amount of it) get tagged and age gated. I imagine it's pretty easy to let advertisers opt out of those areas.
https://www.reddithelp.com/en/categories/advertising/adverti...
What are age-gated subreddits?
Age-gated subreddits are those that are listed as NSFW(18+). We ask that advertisers advertising adult-related products/services focus their targeting on these subreddits to ensure we are serving ads to the appropriate audience.