If you want to apply conditional validation on 'Voter Age' or 'Member Age' then need to add this input greater than or equal to '18'.
{{userForm.controls?.admissionAge?.errors?.greaterThanEqualTo.message}}
If you want to apply conditional validation on 'Member Age' then you need to add 'Admission Age' field greater than or equal to '18'.
Conditional Expression with type 'string'
{{userForm.controls?.memberAge?.errors?.greaterThanEqualTo.message}}