Only when it's safe !
When can a person LEGALLY go to the beach without it's parent?
The beach can be a dangerous place. Permission to go by yourself depends on how your parents assess your ability to handle various situations as well as your personal behaviour.
It's a matter of parental responsibility. Legality doesn't come into it. Perhaps a better attitude would be to understand why your parents don't want you to go alone and see if you can do something to alleviate their concern.
18 without it's parent(s). But if it's a dog or cat, it still needs it's owners.