Hello guys!

I've been struggling with a validation rule. There is a field called "Birthday" where the user populates his/her birthday. When the user populates this field, then it has to allow only ages higher than 17 years old.

For example:
User's birthday: 02/02/2005
Today's date: 15/05/2019
So it means that the user's age is bellow 18, hence it is not accepted. 

How can I do it? I've made it doing the following code, but it didn't work as I wanted:

(TODAY() - DadosPF_DataNascimento__c < 18*365)

I did other things, but there were incorret datas that were allowed instead of being denied. Could you guys please help me? 
 

Hi. I have a test class that makes a mock REST API callout . Now,I need to make another callout in test class and the response body would be response received from first mock callout. The first callout is made and correct response is returned . However,I am unable to use the response returned as body for another mock callout. This comes out to be null. How can I accomplish this? Has anyone encountered a scenario like this.