Head coach Simon King said: "Thomas is certainly a player who leads by example and had some leadership responsibility in our game against Australian Schools XV in December. We have a number of leaders in our group and the intention is to rotate the captaincy during the forthcoming games which make up the Five Nations festival."
Thomas is the son of Cardiff Blues Director of Rugby Dai Young, the former British & Irish Lions and Wales prop.
Tom Prydie was due to have played for Wales Under 18 this weekend, but has stepped up to win his first full cap for Wales. King believes Prydie's elevation to the senior squad can only help inspire
the rest of the squad.
"Tom's involvement with the seniors has obviously been fantastic for his development, and a massive incentive to the rest of our players," he said.
"All of our squad and management wish him well for his international debut on Saturday and we hope to welcome him back into the Wales Under 18 set-up for our remaining matches."
Following this Sunday's game against the Scots, Wales Under 18 take on England at St Helens, Swansea on Sunday, 28 March (2pm), followed by Ireland and Italy at Llandovery on Friday 2nd and Saturday 10 April as part of the Under 18 Five Nations festival (both games will kick off at 2.00pm).
Wales Under 18 v Scotland, Sunday 21 March, 3pm, Melrose RFC
Harley Thompson (Blues); Harry Robinson (Blues), Luke Williams (Scarlets), Sam Campbell (Blues), Eli Walker (Ospreys); Matthew Morgan (Ospreys), Jonathan Evans (Dragons); Rob Evans (Scarlets), Kirby Myhill (Scarlets), Will Griff John (Blues), Cory Hill (Blues), Luke Hamilton (Scarlets), Ellis Jenkins (Blues), Thomas Young (Capt, Blues), Dan Baker (Ospreys)
Replacements:Jack Dando (Blues), Darran Harris (Blues), Samson Lee (Scarlets), Matthew Screech (Blues), Callum Thomas (Blues), Tom Habberfield (Ospreys), Joseff Griffin (Blues), Leighton Bellamore (Dragons)