Unless a kid's a soccer prodigy, they're almost undoubtedly going to being playing at their age level. So basically, kids on a U10 team are almost always going to be 8 or 9, U12 teams will be made up of 10 and 11-year-olds, etc. I can count the number of kids in our program who "play up" on one hand and have fingers left over. We sometimes have 14 year olds on our U19 team, but that's because we don't have a U16 team some years.
One of the very first posts I made on this board dealt with this topic, and my attitude remains the same - while I strongly prefer kids play with kids their own age and skill level, you have to have an age grid that allows flexibility. I don't like 12-year-olds on a senior team, honestly, but I accept that there are reasons why those kids should be there.