Hell, here are all main cast members' ages at the time of the introduction into their respective shows (SGU actors are bold)…
Beau Bridges: 63
Don S. Davis: 54
Robert Picardo: 54 (in Season 5, previously a recurring role)
Robert Carlyle: 48
Richard Dean Anderson: 47
Ming-Na: 45
Ben Browder: 42
Louis Ferreira: 42
Joe Flanigan: 37
David Hewlett: 36
Paul McGillion: 36 (in Season 2, previously a recurring role)
Torri Higginson: 34
Claudia Black: 33 (in Season 10, previously a recurring role)
Rachel Luttrell: 33
Christopher Judge: 32
Amanda Tapping: 31
Corin Nemec: 30
Alaina Huffman: 29
David Blue: 28
Brian J. Smith: 27
Jamil Walker Smith: 27
Michael Shanks: 26
Jewel Staite: 26 (in Season 5, previously a recurring role)
Jason Momoa: 25
Rainbow Sun Francks: 24
Elyse Levesque: 24
It's true that SGU contains some of the youngest actors/characters in the franchise. With two reservations: 1) It's a trend that actually started with SGA (especially Francks, Momoa, Staite), 2) SGU also contains some of the older actors/characters in the franchise. If anything, it's unusual that SGU introduced so many "young" characters at the same time.
(On the other hand, SGU has the distinction of featuring the oldest female main cast member in the history of Stargate. Amanda Tapping, the previous record holder, was 42 when she left at the end of the 4th season of SGA.)