My mom's only child but I've got 6 siblings...5 older and 1 younger (pops was married before, split, remarried/split, remarried etc).
Being an only child, I definitely don't mind being alone whereas my wife hates being in the living room by herself. I can go into the spare and play video games for a few hours like it's nothing....go on long walks, runs etc, no big thing.
Getting closer to my half-siblings now.
Only children can be more mature or less mature depending on how they got raised. It was mostly me and my mom. She didn't have the $$$ to spoil me except at income tax time so I (thankfully) didn't grow up a brat. Plus her and I had similar personalities...so we didn't really clash. I wasn't out partying and doing stupid shyt in high school (saved that for college lmao)....got A's, played PS1/PS2 and worked out.
Being OC does make you more independent, especially with a single working parent....so you have to learn to work with others.