The youngsters from the two cities produced a wonderful game, with Cardiff just edging out their counterparts from Swansea to win 12-7.
Cardiff set the tone for the game by taking the lead through a try by wing Crabbe after just 45 seconds, a score which was converted by outside-half William Hayward.
Swansea fought back vigorously and their captain Chay Smith also went over for a first-half try which outside-half Thomas converted.
With the game locked at 7-7, the next score was always going to be decisive.
In the end it came Cardiff's way, with scrum-half Michael Hale going over for a try.
The two sets of players were commended for producing an excellent spectacle by WRU chief executive Roger Lewis.
"This was a wonderful game of rugby and it was a privilege to see it," said Lewis.
Western Mail head of sport Paul Abbandonato, who helped hand out the trophies, said, "The two teams should be proud of themselves for producing such a thrilling match. As always in a final, there has to be a winner and a runner-up. Cardiff deserved their triumph, but the Swansea youngsters played a full part in what was an excellent advert for age-group rugby."