Age doesn't matter in a concert, but it certainly does in a full staging. Remember that Effie and the other Dreams are teenagers when the show opens (and only in the late 20s at the end of the show). Lillias is in her early 50s, so, no she has no business playing Effie at this point.
Not to mention, even in her prime, she was far from the best Effie. I've seen about a dozen in the past 25 years and she doesn't even make my top 5.
If you're going to cast someone too old for the role, you might as well grab Holliday, McGirt-Nixon, Fuchsia Walker or even Sharon Brown, all of whom were better Effies than White.
But all that aside, they should go for talented newcomer. Holliday was 21 and had only one Broadway credit when the show first came around -- they should find some one like that. Condon did a nationwide talent search and apparently auditioned over 700 performers for the role of Effie for the movie -- maybe one of them would work (heck maybe they should grab a couple of them -- given how demanding the role is and how often several of the actresses during the original run and revival missed performances because they had blown out their voices, you always need two or three understudies for the role of Effie).
"What a story........ everything but the bloodhounds snappin' at her rear end." -- Birdie
[http://margochanning.broadwayworld.com/]
"The Devil Be Hittin' Me" -- Whitney