Dutch. I was mid-twenties and she was mid-eighties. She taught me a lot about life, and I gave her rides to the grocery store. She had been a serious wild child in her day.
She’s been gone almost ten years and I still miss her almost daily.
Almost all my friends are younger than I am. Youngest is 25, which makes her exactly half my age. We traveled through England and Ireland together for two weeks and were still speaking at the end of it, so I think that makes us good friends. 
My best friend at work is 20 years older than I am. Other than the fact that her oldest child is only two years younger than me, I don’t notice the age difference at all.
My oldest friend is 52, which makes him 13 years older than me. We met at work almost ten years ago and clicked, and when he introduced me to his wife (who is 50) we clicked, too – these days, she and I are actually better friends than he and I are.
I have no friends who are significantly younger than me. In fact, I can’t think of anyone I hang out with on a regular basis who is younger than me at all. I do have one work-friend-bordering-on-real-friend who is 9 years my junior, but he doesn’t count yet. 
Most of my friends are my age or 1-5 years older.
I recently started to become friends with a woman who is 21 years older than me, but our personalities wound up being incompatible. Some of that might have been due to her age, but I think most of it was just her.
My friend Wally is about 45 years older than me, now in his mid-90s somewhere. We became friends doing politics together. He is a far right wing libertarian and I’m an FDR liberal. We thought our condo association was being badly run. Neither of us live there any longer but we get together a few times a year and annoy each other about national politics. He left the Republicans in the early 60s because they were too moderate. He isn’t getting back together because they are too racist for him. He is an economic old schooler.
Only a 7ear gap for me. Which is alot considering i was a child.
She was the sister of my best friend, and eventually a very good friend of mine
Okay, maybe I just thought it was more because you’re so much more mature than I am. bats eyes
If aquaintances count, I’d say somewhere in the 25 years range. I’m 45, my band has our own studio where we rehearse, jam, hang out every Tuesday. One of the regulars is a kid just out of college who works for my brother-in-law/drummer. He’s a good guy and I like him a lot, but it’s not like we’re hanging out at each others houses on the weekends.
Other than that … I was always the oldest among my circle of friends for some reason. For the longest times, most of my friends were at least 5 years younger than me. My nickname through most of my late 20’s / early 30’s was “Daddio” as in “Dad”, as in “Ha ha, you’re old.”
It depends on how you define a friend, but I think I’d be safe to say 23. And that’s another Doper. I’m not sure how old his parents are, but I consider them friends as well.
When I was 4 my parents and I became good friends with a man who lived near our house. He was 94 when I met him, had no family or close friends, and his living situation was very poor at the time. I spent many a happy hour talking and playing with him while my parents cleaned up and repaired his house. Lots of good memories of Mr. Palmer, his cats and his tumbledown house full of fascinating things and old pictures. I even used to call him on the phone during the week to chatter at him. Unfortunately his health started declining, and my parents eventually helped him get placed in a nursing home, where he was still living when we moved several states away when I was 8.
90-year age difference. I win!
Currently, I’m friends with several people in their 70s and I’m 25 - so up to a 51-year age difference. They are mutual friends of my boyfriend (who is 42) and I, although he knew them first, met them through the coffee shop where we both work now.
I’m also ‘real’ friends with an 11-year-old girl (daughter of one of my bosses) and pretty friendly with several of my BF’s nieces and nephew’s little friends - although I don’t call those kids on the phone, I have taken them out to movies etc, and spent a lot of time talking and playing with them at various places.
If we’re going to count young children, then the age difference was about 40 years. I’m not sure it counts as a true friendship. We spent time alone together in the absence of others, but was it hanging out or babysitting? He did declare us to be bestest friends EVER.