I can't speak for the Catholic church, but I was a wedding coordinator in a protestant church for a few years, and I have heard of some churches putting age restrictions on flower girls/ring bearers in wedding ceremonies. The church I worked for didn't have any restrictions, but I did hear from some couples that other churches they talked to did. Usually what I heard was that the kids needed to be at least 5 years old and have at least one parent who was NOT in the wedding party available to take them out of the ceremony if necessary.
I can understand where they are coming from - it's a serious covenant being made, and should be treated as such - but I am a big believer in having kids of all ages fully included in all aspects of church life, no matter how much of a "disruption" they may be. We had two 4-year old flower girls in our wedding, and one of them had a fit just as I was saying my vows. Was it disruptive? Yes, a little. Did it spoil our wedding? Not in the least. Do people still remember that? Nope, not a bit. (Well, her mom still remembers, but she was the person most frazzled by the experience!)