The entire point of experiences being unrated is that they’re either unclassified or are currently violating rules in some way. That is not safe for 13+ and should exclusively be limited to at least 17+.
I understand that many of these unrated experiences are simply old and have nothing inherently wrong with them, other than having an outdated rating, but the safer option is to leave it at at least 17+, especially in the cases where the experience has become unrated for some violation. Perhaps Roblox could go back to some of these more popular experiences and update their ratings manually, or maybe certain developers will look to recreate some of these legacy experiences.
(Although, any experience that Roblox modifies the age rating of will have to have the rating automatically reset, if the experience every gets updated again.)
It really should be 18+, rather than 17+, to avoid any potential issues.