Sorry, I think I didn't express myself properly. I know that we can use the simple present with state verbs but my question was about the use of the simple present in sentences with 'since clauses'. We normally should you a perfect tense with "since" and not a present in the main clause, so why in the examples hereunder is the present permitted?
E.g. Why is "My mother looks younger since (= now that) she dyed her hair" possible and not "My mother has looked younger since she dyed her hair"?
The same for:
a) Since when do you have the right to tell me what to do?
b) My kids think that the cell phone is the greatest thing since sliced bread.
c) She doesn’t come to see us since she got married / her marriage.
Is the simple present possible with "since" when we describe (a) a present fact or (b) a habitual present occurrence?