Iain Henderson returns in the second row to partner Michael Kearney, while in the back row Dominic Gallagher comes in at openside with Jordi Murphy moving to the number 8 position.
The three changes in the back-line see the return of Ulster's Paddy Jackson and Luke Marshall at out-half and first centre respectively, while Andrew Boyle will start on the left wing.
Speaking from the team base in Italy, Ruddock said: "Following two very physical battles, we have turned our attention to a Scotland team that is much improved from the Six Nations.
"They played very well against England and play a very expansive offloading game. We will have to be at our best to beat them and that is our goal.
"I have made five changes to the starting line-up with experienced players like Luke Marshall, Paddy Jackson and Andrew Boyle coming back into the team."
IRELAND UNDER-20 Team & Replacements (v Scotland Under-20s, IRB Junior World Championship Pool C, Stadio Mario Battaglini, Rovigo, Saturday, June 18, kick-off 8.10pm local time/7.10pm Irish time):
15 - Tiernan O'Halloran (Galwegians/Connacht)
14 - Andrew Conway (Blackrock/Leinster)
13 - Brendan Macken (Blackrock/Leinster)
12 - Luke Marshall (Ballymena/Ulster)
11 - Andrew Boyle (UCD/Leinster)
10 - Paddy Jackson (Dungannon/Ulster)
9 - Kieran Marmion (UWIC/Exile)
1 - James Tracy (UCD/Leinster)
3 - Tadhg Furlong (Clontarf/Leinster)
4 - Michael Kearney (Clontarf/Leinster)
5 - Iain Henderson (Queen's University/Ulster)
6 - Eoin McKeon (Galwegians/Connacht)
7 - Dominic Gallagher (Dublin University/Ulster)
8 - Jordi Murphy (Lansdowne/Leinster)
16 - David Doyle (UCD/Leinster)
17 - Conor Carey (Ballymena/Ulster)
18 - Daniel Qualter (Buccaneers/Connacht)
19 - Shane Buckley (Garryowen/Munster)
20 - Peter du Toit (UCD/Leinster)
21 - James McKinney (Queen's University/Ulster)
22 - Craig Gilroy (Dungannon/Ulster)
IRELAND UNDER-20 POOL MATCHES (all kick-off times are local time) -
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, 8.10pm: Ireland v Scotland, Stadio Mario Battaglini, Rovigo