a joint account is an account that is joint together for an opening account. While beneficiary account are people that gain some promo from the bank

Yes. If you want to get out of a joint account, you can contact the bank and submit a written request to be removed as a joint holder of that account. The other parties involved in the joint account have to approve your removal from the account, only then the bank will complete the formalities.

Full ownership of a joint account passes to the surviving joint owner unless the joint account was set up for purposes of convenience only and the account is otherwise devised in a will.

The surviving joint owner is the sole owner of the account and can maintain it or close it. That is the reason for having a joint account.

No. Two minors can not open a joint account