How to extract Date Of Birth date value from 3 separate textfields - Date, Month, Year?

For DOB (Date Of Birth) I have three DropDown textfields in my signup form:

How can I pass a singular date value from these 3 separate textfields into my supabase public.user_data 'dob' (date) column?
Also, how can I calculate the Age value based on selections in these 3 fields to validate the "Create Account" button before the rest of the data is passed into database?