Depending on where you are getting stream information to start with, you probably already have the data.
Both channels and streams API return a mature tag with false/true.
EDIT: actually, there might be a null instead of false.
https://github.com/justintv/Twitch-API/blob/master/v2_resources/channels.md#get-channelschannel
https://github.com/justintv/Twitch-API/blob/master/v2_resources/streams.md#get-streamschannel