Had clockwork periods since puberty for 5 years . now hardly get it. doc. mentioned pcos but doesn't that begin right at puberty? can it happen later?

Yes it can: Yes it can happen later especially if you have gained some weight. Some women have a critical threshold weight that, once they exceed that number, their periods become irregular. Other things to consider are your thyroid status. A simple blood test will tell you if you are hypothyroid. I'm assuming you've already ruled out pregnancy with a test.

PCOS onset: PCOS onset is usually after the ovarian development has aged enough to produce the multiple cysts seen under hormone stimulation. That is usually not puberty but later.