Weddingbee boards

Is there ever a time it's appropriate to invite some kids and not others?

I have several friends and family members that have children and are coming from out of town and I have no problem inviting those children whatsoever.  I'm wondering more about the handful of guests friends of friends that have kids in the 10-13 & 18-25 age range that live in town.  The only reason I'm second-guessing them at all, is because of numbers.  I plan of having babysitters on hand so the kids can go into the house and hang out away from the reception if they want, but when it comes to seating, I'm concerned I'm just not going to have enough room.  I'm having a backyard wedding and getting married on a small point of land on the shore of the lake.  Every square inch counts.

Is there a cut-off age for which you don't need to logically invite someone's children, especially if you're not close to them and just want their parents to have a night out away from the kids?

I think it depends on the relationship with the primary guest.  What I mean by that is you may consider inviting a 10-year-old for example, because you're really good friends with his mom and feel rude leaving a child who would need childcare out of the wedding. 

However, when it comes to a "child" who is say, 20 years old, if I was only close to the parent, and not the child, I wouldn't include them. At that point, they are an adult and according to most etiquette standards, would be receiving a seperate invitation.  If your relationship with them isn't close enough to warrant a seperate invite, I think it's fine to leave them out.  I hope that makes sense ;)

@LucyMegs140: ok that makes sense for the older kids.  The other ones I'm referring to are both around 11 years old and their mothers are two friends of mine.  So it's not like it's a couple with a child and I'm just excluding the child.  I'm friends with the child's mom but don't know the kids well...