I have three brothers, one of whom is three years younger than me, who was my best and closest friend growing up. We eventually drifted apart when he was around 12 and I was around 15.
We did basically everything together, to the point that I think it actually arrested my development a bit -- an 8 year old spending time exclusively with a 5 year old, a 10 year old spending time exclusively with a 7 year old, and so on. I had only one friend my age and I didn't spend as much time with him as I did with my brother and his friends, who were all substantially younger than me.
My other two brothers are eight and nine years younger than me, so we weren't really close as I was already a teenager when they were children.