I'm back playing this again.
These are the levels you can turn in and get gold flags. It claims you can prestige for 1 golden flag starting at level 23, but it lies and will not work until level 25.
1 25
2 28
3 32
4 35
5 37
6 39
7 40
8 42
9 43
10 44
11 46
12 47