DOB by Age field

Hi Everyone,
As you check the below screen I have created a new filed “Age” in Add patient form by form layout. I want to fill DOB field automatically when I put age number in “Age” field. Month and Day should be current in DOB field but Year should be according to “Age” field. Anyone please suggest the code. I would be really thankful to you.

Regards,
M Sheeraz Shaikh.