It has been made clear that they are neighbor friends, but not same age friends. Consider it to be equivalent to not inviting a boy to an all girl party. They can still be friends, but they may not have friends in common.
Don't worry so much about it. Your daughter will be making lots of friends her own age and will not be so in need of the friendship with the neighbor.
Do you have an older brother or sister? Know how they love to play with you until
a friend their age comes over? Does that mean that your sibling doesn't really like you?
No, not really.
If it really is bothering you, ask the mother why your daughter wasn't invited to her daughter's B-day. Maybe she thought that she would feel uncomfortable around a group of girl's that are older. Or maybe it was a cost issue. Maybe the daughter really does like your daughter, but didn't want her friends to think she played with "babies".
The pressure to fit in can be terrible at that age.
Perhaps, just say that you're planning a party for your daughter and you would like to invite her daughter. But, although they get along well, you weren't sure if she would be comfortable being around a group of girls that are younger than her. What does she think? Should I send the invite to your daughter? GOOD LUCK!
Hope this helps some.
S