The Ospreys were in scintillating form in the first half of the game and swept into a 19-point lead courtesy of tries from scrum half Martin Roberts, centre Chris Tossell and Wales U19 captain and full back Jonathan Spratt.
However, things were turned on their head in the second half as centre Emyr Williams crossed for a brace of tries for the Scarlets and Roger Davies, the outstanding full back, adding a third to bring the Scarlets to within touching distance. The Ospreys struck back and appeared to have won it when James Hook crossed for a late try, but Daniel popped up in stoppage time to win the day 25-24.
The Scarlets U16 were also successful, just as their U18 had been seven days earlier when they defeated the Ospreys 36-5. Blues South battled hard against the odds in another excellent game, but a Lee Collier try was never likely to be enough to deny the Scarlets a hat trick of titles.
Daniel Evans, who will step into the Scarlets academy next season, stepped up to the mark on the day with a try, conversion and four penalties as the Scarlets won 19-5.
Reebok ambassador Rob Howley said, "Both were excellent games and to be honest we have seen two weekends of outstanding rugby. I think the future of the game in Wales looks particularly good, we seem to be bringing through a whole host of outstanding youngsters."
With the U18 title also clinched by the Scarlets last weekend, they have achieved a unique treble in the opening season of the WRU Reebok Regional Championships competition.