I dunno, I did it at 60, may be a bug,, or "not intended", but i have used it for 4 teams now at 60, no problems at all.
What i do:
1. Level 4 toons raf'ed to 59. Work 90% of the way to 60.
2. Turn in 1 extra quest on my "last guy" dinging him to 60.
3. Immediatly gift from the level 60 player, backwards down the chain.

I end up with 3 toons at 60 & 90% of the way through that.
Then I proceed to gift up some level 30's I had. (all from Level 60 toons)

Works beautifully

Working on my 5th time up.