In Annett's absence, back rower Jordi Murphy will captain the side.
Andrew Conway, who is one try away from equalling the competition's top try-scoring record of 10, moves to full-back as Craig Gilroy returns to the back-three. Tiernan O'Halloran is the player to make way.
Alex Kelly and the 18-year-old JJ Hanrahan will form the centre partnership, with James McKinney slotting in at out-half alongside Kieran Marmion.
In the forwards, Tadhg Furlong, who is also 18, is joined in the front row by Finlay Bealham and David Doyle, who started at openside flanker against South Africa last time out.
Daniel Qualter will start in the second row with the in-form Iain Henderson, while Shane Buckley and Aaron Conneely are restored to the back row.
Speaking ahead of the game, Ruddock said: "We looked fatigued last time out against South Africa and so I have made a number of changes for our fifth game of this tournament. Another short turnaround see us fighting it out for seventh spot with Wales.
"There was nothing between the two teams in the Six Nations earlier this season and both teams will be keen to end the tournament with a win."
IRELAND UNDER-20 Team & Replacements (v Wales Under-20s, IRB Junior World Championship 7th-8th Place Play-Off, Stadio Comunale di Monigo, Treviso, Sunday, June 26. kick-off 12pm local time/11am Irish time):
15 - Andrew Conway (Blankrock/Leinster)
14 - Craig Gilroy (Dungannon/Ulster)
13 - Alex Kelly (UCD/Leinster)
12 - JJ Hanrahan (UL Bohemians/Munster)
11 - Andrew Boyle (UCD/Leinster)
10 - James McKinney (Queen's University/Ulster)
9 - Kieran Marmion (UWIC/Exile)
1 - Finlay Bealham (Belfast Harlequins/Ulster)
2 - David Doyle (UCD/Leinster)
16 - Jonathan Murphy (Banbridge/Ulster)
17 - James Tracy (UCD/Leinster)
18 - Michael Kearney (Clontarf/Leinster)
19 - Eoin McKeon (Galwegians/Connacht)
20 - Peter du Toit (UCD/Leinster)
21 - Paddy Jackson (Dungannon/Ulster)
22 - Brendan Macken (Blackrock/Leinster)
IRELAND UNDER-20 RESULTS -
Friday, June 10: England 33 Ireland 25, Stadio Comunale di Monigo, Treviso
Tuesday, June 14: Ireland 26 South Africa 42, Stadio Comunale di Monigo, Treviso
Saturday, June 18: Ireland 30 Scotland 13, Stadio Mario Battaglini, Rovigo
5TH-8TH PLACE PLAY-OFF:
Wednesday, June 22: South Africa 57 Ireland 15, Stadio Plebiscito, Padova