I happen to be a fan of Lea Michele's voice, though not so much her acting, and am not going to speculate whether or not she can carry as big a show in as big a theater as Wicked. If it is true, however, she is not too young to play the role if she has the voice for it. Just this week in Time Out NY, Lisa Brescia said (adorably!)in response to the question "how old are you?", "Elphaba is 19, so for me to say might not be wise". Everyone's just used to seeing older Elphabas and Glindas.
"I thought that that was just going to be a like one shot deal for me, you know, but they kept talking about it like, 'when we do the production, when we do the production', and I was like, 'yes, yes, when you do the production, cool, I will come and see it'". - John Gallagher Jr. on SA