Certainly. Some of our earliest testers were pre-teens.
To use the website you do need to be able to read the subtitles and text so we wouldn't recommend it for under 8s, but the content is easy to use and follow and suitable for learners of all ages. You will need to ask an adult to complete the payment screen; they may prefer to set up your membership for you. At BSL Online the only information that we ask you to provide is your name and email address.
We recommend that you follow online safety guidelines like these, and don't share personal information online.
For parents, you may want to look at guidance here