Hi - the "baby" in my husband's family is nine years younger than he is and six years younger than the next youngest sibling. The "baby" is now 21 and has a beautiful relationship with my husband and with their oldest sister who is 14 years older than the baby. The two older kids played with her a lot and did a lot of diaper changes and taking care of her as a baby. I think it has been incredibly positive for her because she had two "grown-up" siblings when she was in highschool which is a difficult time anyway, plus their parents were going through a divorce. She spent several summers living in our guest room while she was a teenager and she knew she had extra grown-ups in her life who could help support her. I think it was so helpful to her as a teenager to have these two older siblings who had enough life experience to actually give her some good guidance, but who were still young and "cool" enough that she wanted to listen to them!
She is now just the sweetest, most capable, most grounded, most responsible 21 year old I know. I really think a lot of that had to do with her siblings. She does have a lot of memories of them playing with her and the nice thing is that they are all positive. The older kids used to argue about whose turn it was to play with her. For me, I have a lot of positive memories of playing with my older brother (4 years difference) but I also have a lot of memories of getting on his nerves and him not wanting to play with me because we were close enough in age for me to be a nuisance! I really think the larger gap makes it easier to have a good relationship later in life.
One other thing I would say is that my husband's sister got to become an aunt when she was young enough that her nephew was almost like a younger brother to her. They are six years apart and have a very close relationship. I can see that she is going to get to be the "cool aunt" for him and for some of the other nieces and nephews who are closer to her in age because she's so much younger than their parents. She also just got engaged and she has the whole family excited and trying to figure out how to help her because everyone else is in a place where they have the time and/or money to help support her in life. I think being the baby has really given her a lot of security in every sense of the word.