How do I make an option in Dropdown disabled as well as the default option?

Example:

Age: (option 1) Select age

        (option 2) 17

        (option 3) 18 ...and so on.

I want option one to be my default option but when the user is filling my form, he is not able to select that as his option.

If I understood you correctly, you don't want to allow users submitting the form without filling the age. However, "Select age" is already filled in your dropdown field so users can submit forms with "Select age" value.  

You can use conditions to prevent users submitting "Select age" value.

I cloned your last modified form to my side and I checked the "Age" dropdown field in your form. My suggestion is to "Hide" submit button until users select a valid age and giving a message "Please select your age!".

Please follow the steps below:

1. Add a "Text" field to your form and write your message in it.

2. Click "Conditions".

3. Select "Show/Hide" Condition.

4. Change the visibility of "Submit" and "Please select your age!" fields with "IF" conditions. Please have a look at the video below.

I provided you an example form in the following link. You can clone this link to your side.

https://form.jotform.com/63013130470944 

If you need more details about how to clone an existing form to your side, please follow the guide: How to Clone an Existing Form from a URL 

If it is not the issue, please provide more details about your problem.

If you need any further assistance, please feel free to let me know.

Thank you.