Cardiff Blues Under 20s saved their best to last but it was not good enough to help them through to the final of the WRU Regional Championship.
The Blues beat the Scarlets 44-33 at Treforest but the Scarlets booked their place in the Millennium Stadium April 30 final courtesy of a bonus point for scoring four tries.
The Blues tries came from Tom Riley, Nathan Trevett (2), Tom Dickens, Dafydd Hewett and Chris Clayton with outside half Dai Flanagan kicking five conversions and a penalty.
Wings Steven Davies and Luke Marsh, captain Ian Jones (2) and outside half Craig Evans scored the tries for the Scarlets with Evans adding four conversions.
The Ospreys defeated the Dragons 30-14 at Aberavon to secure their safe passage through to next months final at the Millennium Stadium.