I just ran through the list and calculated it.
For Round 1, with Dugan out, and assuming Farrell and Beale are the centres (not Runciman), our lineup will be something like this:
1 Quinlan (21)
2 Morris (27)
3 Beale (23)
4 Farrell (22)
5 Nightingale (27)
6 Widdop (24)
7 Williams (22)
8 De Belin (22)
9 Rein (23)
10 Hunt (27)
11 Frizell (22)
12 Thompson (25)
13 Merrin (24)
14 Cooper (25)
15 Stockwell (22)
16 Ah Mau (24)
17 Stanley (22)
The average age of that lineup is 23.6 (probably closer to 24 because I only looked at years, not days).
When you include the rest of our top 30 (Dugan 23, Harrison 28, Runciman 20, Mata'utia 23, Garvey 20, Witt 30, Matthews 25, Green 21, Groat 21, Bird 18, Glymin 20, Ailaomai 22, Mataka 25), our squad's average age is 23.2 (again, probably closer to 23.5).