There is no definitive answer to this question, as it depends on the individual and what they are using Tumblr for. For some people, Tumblr may be appropriate for users as young as 13, while others may find it more appropriate for users over 18. The key thing to keep in mind is that Tumblr is a social media platform that can be used for a variety of purposes, so it's important to use caution and common sense when deciding what is appropriate for each individual user.