Having read wmshub's report, which can be found here http://www.boardgamegeek.com/thread/91031, I decided to try the game with my 4 year old daughter.
She was quite excited as she has seen her Mum and Dad play the game, but hasn't been allowed to join in before.
I decided not to bother explaining all the rules ahead of the game as I was concerned she might be overwhelmed by the scoring process. I also played without farmers as they are quite a tricky concept to fully understand.
She loved the whole tile-laying process, and was placing meeples pretty much anywhere she felt, with a bit of correction from me. After a few turns, she seemed to have the idea of meeple placement, and was managing a few tricky moves to close off cities or to complete roads.
Scoring was pretty tight throughout, but if it wasn't for my lucky placement of a tile to finish off three seperate cloisters at once, I may well have lost the game. Final score 87 for me, 77 for her. I was pleased with her score, as it's quite high for a two player game. I'm not sure how much difference playing without farms makes to the expected score, though.
She's already asked if we can play again, but this time with Mum as well. Hopefully we can gang up on her....