I use the iBert and can envision my daughter outgrowing it at around 4 years of age.
For what it's worth, I once had a very enlightening conversation with another parent at a playground (he saw me ride up with my daughter on my bike).
He said that he had a similar setup (with the carrier in front) and one day when he tried to put his son into it, he had grown too big and couldn't fit.
He said he suddenly felt really sad that he would never be able to have his kid ride in front with him, never be able to talk to him in the same way while riding, or point out the different things they could see while riding together.
He ultimately switched to a trailer, but told me that it was never the same.
I have not reached that point yet, but I know exactly what he means.