1. They are calm in a crisis. Youngest children have the benefit of growing up watching their older sibling(s) make mistake after mistake, and come back from it. Youngest children aren’t the type to freak out or have a knee-jerk reaction, they know failure or disappointment is natural, and just the starting point to something better.
2. They’re adventurous. Youngest kids grew up trying to hang out with their cool older brother/sister and their friends. To do this, they had to prove they weren’t scared, that they could do things kids their age weren’t doing yet. They’re used to taking a leap and figuring out where to land on the way down.
3. They enjoy observing action more than being involved in it. Thankfully this applies to drama. While they love reality TV they are calm in real life. When the two of you fight they seek consensus and to solve the issue at hand rather than to cause a bigger, louder fight by simply trying to prove themselves right. When it comes to people they love, they believe fighting should not be a zero sum game where one person wins and the other loses, they want you to both feel good at the end of it.
4. They (annoyingly) need you to treat them with kid gloves from time to time. They’re the baby of their family, they aren’t used to being treated harshly. When you need to criticize them or get them out of a rut, you’ve got to do it gently.
5. They aren’t finicky. Youngest kids get hand-me-downs. They don’t get to be fancy. This has the added relationship benefit of saving some $$$ in relationships since they have reasonable tastes.
6. You realize they’re kind of naive. Everyone looks out for the youngest kid, the whole family. They are used to people having the best intentions and looking out for their best interests.
7. You have a lot of fun with them. Do you know how much crap youngest kids get from their siblings? Not having a sense of humor was never an option. To this day they’re a playful partner, they love to have fun and laugh.
8. They enjoy breaking the rules. Parents tend to be strict with the oldest child but by the time the youngest comes along they get lax on enforcing the rules. Youngest siblings grow up thinking that rules are flexible or that they don’t apply to them. Your partner probably has a mischievous side, indulge it everyone once in awhile by skinny dipping or sneaking into a movie.
9. They are one of the most emotionally intelligent partners you’ve had. Youngest kids are rarely awkward — they grow up with people around them so they learn social skills very early. They also get to observe each of their siblings relationships with each other and their parents before being in that situation themselves, they’re more aware than others about the emotional aspects of relationships.
10. They make a great team. Youngest kids like teamwork, they’ve always been a part of one — their family. They make good partners because they genuinely enjoy being part of a couple, and they strive for both of you to be happy.
11. They aren’t the type not to stop and ask for directions, and you love them for it. Youngest kids grew up around a network of people who were better at everything. If they needed to know how to do something they could ask their older sibling for the answer. They were never expected to be the know-it-alls so it’s easy for them to ask for help.
12. They may be a tad overcompetitive (okay extremely overcompetitive). Growing up, a youngest kid is the star. When we grow up, we have to earn that status through accomplishments — even seemingly tiny accomplishments like winning Catch Phrase.
13. You love that they’re so happy-go-lucky. Youngest kids rarely get their way because they can’t compete with the older, bigger, louder kids, they had to learn to go with the flow and be happy with whatever they ended up doing, even if it wasn’t their first (or second) choice. They’ll defer to you often and be sincere about it — they really are happy to do what you enjoy. 
