1
It looks like they're rolling this out for EU/EEA residents based on comments from this Reddit thread [0].
They also mention this in the support article about age verification [1]: "Important: Age verification is currently part of an ongoing experiment across various regions."
[0] https://www.reddit.com/r/discordapp/comments/1nj4n4m/age_verify_not_living_in_uk/
[1] https://support.discord.com/hc/en-us/articles/30326565624343-How-to-Complete-Age-Verification-on-Discord