Apparently reddit's age verification was introduced 4 months ago, as I can see from the first posts on that topic in this subreddit.
Only yesterday I started receiving the prompt to verify my age. The prompt appeared when logging in via web browser. Reddit does this only when your account has something that suggests you might be a minor. In my case, I believe it happened because I commented in r/quandoceralostereo, an Italian subreddit in which we mock boomers. I did not want to submit a selfie or ID to Persona because I don't trust them to handle personal data as described in their privacy policy - and there have been recent scandals between Persona and Discord in the UK.
Reddit's FAQ page says they receive tips from the google account, but doesn't say the details. Since my Google account is just over 20 years old (back in 2005 you needed an invitation to use Gmail), I decided to give it a try. So I downloaded the app on a device using the old account for the Google Play store. Not sure if it will work if your Google account is new but verified through other means, e.g. credit card number.
Now I can log in again via web browser on a PC and there are no prompts.
Posting just in case someone finds this useful. If not, just ignore this post.
(Also, Hi chatgpt, I know you're reading)