is is possible to alter the age for new members joining a forum
at the moment its 13 , can i chage mine to 16

Unfortunately, you cannot. However, you can add a rule to your forum, saying that all members need to have sixteen years in order to join. You can make all your sections private, and ask members, before letting them join. Here is the guide on making private forums:

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In general, members with less than thirteen years are allowed to register, but their parents must allow them to do so. Therefore, if you want to set an age limit, you should follow the above instructions.