Conditional required validation in angular reactive form

I want to apply conditional validation on some properties based on some other form values. I have referred some answers Angular2: Conditional required validation, but those are not fulfil my need. Because I have to implement conditional validation in 40+ form(around 30 fields) of my large enterprise application. I don't want write the same code in every component and change the FormControl name. I don't know how this can be achieved via Directive.

if age control valuev is greater than 18 than the license number field is required.

Here's my code:

this.userCustomForm = this.angularFormBuilder.group({
age:['',Validators.required],
licenseNo:[''] // Here I want to apply conditional required validation.
});


In my application there are some cases where I want set conditional validation based on nested FormGroup or FormArray values.

Please guide me, how can I achieve this.

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