Now, the teams will go head to head in what promises to be another outstanding Age Grade spectacle.
For the Scarlets, it's also an opportunity to make up for the disappointment of their under 18s who were narrowly beaten by the Blues in last month's final at the Brewery Field.
However, the Ospreys are past masters at winning the vital Age Grade finals and they will hope to redress the balance this season by putting one over on their closest rivals.
Scarlets team manager Wyn Thomas said; "They boys have done particularly well this season and have developed well, boith individually and as a squad.
"We are delighted to be in the final and would love to finish the campaign with one more victory."
Ospreys team manager Graham Court said; "I suppose we are hoping that it's third time lucky having gone down narrowly to them twice this season.
"The competition has been a really good advert for the Age Grade system and fingers crossed, it will finish on a high for us."
Evan Yardley will lead the young Ospreys from hooker, whilst for the Scarlets, flanker Sion Bennett is named as skipper.
Ospreys: R Thomas; R Williams, R Evans, B Thomas, D Skone; S Davies, J Preddy; N Smith, E Yardley (capt), S Mcdonnell-Roberts, D Parry, R Hughes, L Wilson, R Dudley-Jones, A Waite. Replacements: Sam M Davies, B Roberts, C Davies, S Otten, M Davies, T Owen, M Jenkins
Travelling Reserves: J White, R Leonard
Scarlets: W Lewis; D Morgans, S Hughes, G Owens, J Randall; J Williams, C Lloyd; B Davies, T Ball, G Davies, C Marks, D Morgan, S Bennett (capt), D Thomas, J Williams. Replacements: C Evans, D Havard, T Morrissey, T Treharn, D Doughty, G George, S Evans, R James