Some things are younger than you think. It really surprised me to learn that the snowmobile has only been around since 1960. I guess kids today would be surprised that the ATV was only brought to market in the 80s. I remember when the first ones came out - they only had three wheels and would roll over on top of you if you weren't careful.
Which brings me to peanut butter and jelly. Jelly has probably been around for centuries, and peanut butter was patented in 1884. But it took many decades before the two were put together with commercial white bread into that most American of sandwiches.
When I was about six years old, I was babysat by a lady whose kids were about ten years older than me. I know they would have been little in the early 1960s, because she once told me she was potty training her middle son when Kennedy was assassinated. She also once told me that she hadn't learned about peanut butter and jelly sandwiches until after her kids had already been born. She said that when she was little, she ate peanut butter sandwiches but never with jelly. The first time someone suggested she add jelly, she thought it didn't sound very appetizing.
So when was peanut butter and jelly invented? I ran a Google Ngram on the phrase "peanut butter and jelly" and here are the results. For those who don't know, Ngram counts the frequency of a word or phrase in all the books Google has scanned, which is a huge sample of books. For instance, if you run Ngram on Atari, there is basically nothing until the late 1970s, when it explodes. (There's an interesting little bump around 1890 which probably has to do with the word atari originally being something you say while playing Go.)
Going by the Ngram results, peanut butter and jelly was invented some time between 1920 and 1940, but didn't go mainstream until the early 1960s, which would be right around the time my old babysitter learned about it. So it took 80 years from the time peanut butter was invented until the peanut butter and jelly sandwich was well known. It makes you wonder what two currently available foods will suddenly be discovered to go well together 80 years from now.
An unrelated story is that when this babysitter wasn't able to come and watch me, she would send her friend who was an older lady from Ireland. Once I went over the handlebars on my bike and hit my head, and this Irish lady rubbed butter on the bump. Go figure.