To enable your password restrictions for all purchases or after 15 minutes, follow these instructions:

Go to SETTINGS > General

Select Restrictions

 

Tap Enable Restrictions. You'll be asked to enter and confirm a PIN code (Skip this step if you've already enabled Restrictions)

 

Scroll down and tap require Password or Password Setting

 

Tap Always Require or Require After 15 minutes

 

You will be asked to sign in to your iTunes account again.