The only real way of doing this would be to get people to email photo ID and this creates more problems than it solves- a lot of work for itchio, serious issues in the event of a data breach, and anyone who's ever used Photoshop could get past it easily.
Internet companies can't really enforce punishments in the event that someone lies about their age, and each country's laws differ so that throws up even more issues.
One possible idea would be to have an opt-in button for 18+ content and default to not showing NSFW games. It's no inconvenience or real barrier, but it would at least stop children (and adults) who don't want to see these things having to while browsing.