Twitter used to be very clear that you had to be 13 to use their site. In fact, if you look at the image in this post (or at the prior terms of service), you can see that previously, it was the first bullet in the "Basic Terms" section.
On September 18, 2009, Twitter released a new Terms of Service that omitted the age requirement which you can see here https://twitter.com/tos. Additionally, Twitter does not ask your age when creating an account. They also have safety tips for parents that do not mention an age requirement and they announced an age screening policy that indicates Twitter account holders can determine what age you must be for use. This might appear to indicate that there is only an age requirement to engage with certain Twitter accounts.
Our Policy Towards Children: Our Services are not directed to persons under 13. If you become aware that your child has provided us with personal information without your consent, please contact us at email@example.com. We do not knowingly collect personal information from children under 13. If we become aware that a child under 13 has provided us with personal information, we take steps to remove such information and terminate the child's account. You can find additional resources for parents and teens here.
This (If you become aware that your child has provided us with personal information without your consent)
could be interpreted as Twitter requiring parental consent under 13. However, this (If we become aware that a child under 13 has provided us with personal information, we take steps to remove such information and terminate the child's account.) could be interpreted as 13 being the age requirement.
Maybe someone from Twitter can clarify, but in the meantime, what do you think?