When a child turns 18 you are able to transfer the ownership across to them. Depending on how you have structured the investment you may (in your own name) or may not (your name as trustee for child) have capital gains tax upon the transfer.
Turning 18 does not mean you must immediately transfer control. The trustee can continue to control the assets until a point when they are ready to transfer ownership.
With an InvestSMART Professionally Managed Account (PMA) we request a signed letter from trustee and child. The child must then open an account in their own name in preparation for the transfer.