Age Calculator with VueJS

Hey there, thank you for checking out my solution. This is my first time using this platform and I am somewhat new to Web Development. I have a few questions I'd like to ask:

• What would a proper VueJS Web application look like? As in the file structure, code structure, ect..
• How can one effectively approach project planning?
• Also I have an issue with the button not changing color when hovering it, any clue on why?

.calculator-submit button{
border-radius: 100px;
border: 0;
background-color: hsl(259, 100%, 65%);
.
.
.
}
.calculator-submit button:hover{
background-color: hsl(0, 0, 8%);
cursor: pointer;
}

Thank you in advance.