People bring this up a lot in fan chats, so here’s my take from someone who spends way too much time trawling fan galleries and forums.
On the simpler end, most casual fan art sites that host 'Naruto' or 'Konan' fanworks lean on soft age gates: a mandatory click-to-confirm checkbox, an explicit 'I am 18+' button, or a date-of-birth field. Those are lightweight and easy for creators and visitors, but they’re mostly legal theatre — they rely on honesty and cookies rather than cryptographic proof. You’ll also see content warnings, explicit tagging systems, and NSFW toggles that hide images unless your profile preferences allow mature content.
If the site is monetized or carries higher legal risk, expect stronger measures. Marketplaces and subscription platforms often add payment-method checks (credit/debit card or PayPal) or use third-party identity providers like Yoti, AgeChecked, Jumio, or Veratad for document checks. These services verify age with an ID scan or database match. Streaming or large hosting platforms may combine IP/geolocation filters, CAPTCHAs, social login gates, and paid verification to avoid hosting problems. Personally, I appreciate when creators and platforms take extra steps — it makes browsing mature 'Naruto' fan art feel less sketchy and protects younger fans, which I care about when sharing anything involving characters I love.