Who can I appoint as my beneficiary in my Will?
There are no age restrictions for Beneficiaries.
However, Beneficiaries under 21 years old cannot receive your assets directly. The gifts instead will be managed by an adult until they become of age. Your Executor by default will serve this role unless you indicate otherwise (e.g. the minor child's Guardian). You can indicate this in Select Clauses > Customisable Clause.
Your beneficiary can be anyone including your executor / guardian / trustee. The only restriction here is that your beneficiaries cannot be witnesses to your will. The term "beneficiaries" includes all beneficiaries, trust beneficiaries, remainder beneficiaries, and any substitutes for these beneficiaries in the Will.