Broadway and theater in general doesn't pay much attention to ages.... just recently there was a thread about a 20+ actress being hired to portray Anne Frank.
...in the new Fiddler revival, the daughters who are getting husbands should be late teens and the actresses hired are in their 20's-30's. It would be unheard of for orthodox daughters to wait until that age to wed.
-when we saw Ms. Honey is Matilda, Lauren Ward portrayed her at 40 years old..while her replacements have been correctly younger and younger which suit the role as a teacher trying to find her way...
While it would be maybe appropriate to have a younger Eliza, i don't think it's a requirement of the role..and while my personal preference might be to see or not see Kelli in the role, I don't think it's a wrong choice.
Kelsey Grammer - not sure about that one...