It sounds like you have an exceptionally sociable little girl for an 18 month old.
This makes it hard - because most other 18 month olds are still at the point of either wanting to play alone, or just play alongside other children without much interaction (which is very normal for that age group). And older kids who are fully mobile and more advanced communication wise don't want anything to do with "a baby".

Don't take it hard - your DD doesn't. Once her communication skills are more developed older kids will want to play with her.
My son is 2.5 and my daughter is almost 5. In the last three months my son has started speaking in full sentences and in that time, my daughter has been *much* more willing to play with him, and include him in her games. Before he could talk she had zero patience with him.

It's ok, don't worry.