There's a new book out called "Birth Order: What Your Position In the Family Really Tells You About Your Character". And it lays out the most compatible people for you to date, based on their birth order. Check this out
First-born with last-born works. Oldest siblings are more likely to want to take care of people, and youngest siblings want to be cared for.
Only child with youngest child works. People who are the youngest in their family inspire creativity and freedom in people who didn't have any siblings.