Probably several books; looking around, I see a copy of Van Vogt’s Destination: Universe that I got in 1964. I also have a few of my old “All About” books (All About Electricity that’s copyrighted 1957 and which may date from that time.
Books. And more books. I have at least one that I bought in 1955. My wife has an original American edition of Lord of the Rings with no copyright notice, bought in 1959. I have some vinyl records from the late 50s. And nearly a complete run of Scientific American from around 1965 to the October, 2013 issue. I might have some older books, but I can’t think of any offhand. Oh, and my PhD thesis from 1962.
I had to check to see if I had posted this - my oldest possessions are two stuffed animals, one a Snoopy (which was complete with aviator goggles at one point) and an ancient black teddy bear. I’ve had them since I was three at least, 43 years.
I have a baby blanket my dad’s mom made for me in 1982 (she’s never been a “grandma” to me at all but she is crafty) and one or two of my newborn clothes. I have them because my mom saved them for me. Neither are super special to me but I wouldn’t get rid of them.
I have a tiny, angry teddy bear that I’ve had for over 50 years, since the day I became old enough to want a teddy bear. No doubt that occurred when I was 3 days old :D.
I have a bookcase that was my great-grandmothers. It was made from wood from some other piece of furniture that well predates that.
Feasts for All Seasons, a cookbook by Roy Andries de Groot. I’ve had it since 1967. Lots of interesting recipes (someof which I’ve actually tried) and information.
Just of curiosity, the OP asks what item you’ve had for the longest time - so why are people answering with the oldest item they own?
My birth certificate. It’s 55 years old, yellowed a bit, looks like something out of the early eighteen hundreds. I’m eligible to be president!
boy scout pocket knife.
My first pair of real 14k gold jade earrings that my father bought for me when I was about 15.
He told me I couldn’t get my ears pierced, he was adamant that I couldn’t get my ears pierced. Only pagans had pierced ears. (The cousins on my non-Catholic side of the family couldn’t get pierced ears because only those damn Catholics got their ears pierced :rolleyes:).
So being the typical teen, I took $5 and walked to the doctors office and got my ears pierced. My father was pissed but once the ears healed and I could take out the starter earrings he took me shopping for a nice pair.
I bought a tapestry when I was 16 from one of the local head shops. It’s so old and fraying a bit around the edges but I still love it. I borrowed $20 from my father to get it and he liked it so much he told me I didn’t have to pay him back.
I think it would be the 1959 version of the APBA baseball. it is a dice based game of baseball designed to recreate accurate statistics. They still produce the cards each year, though most play it on computer these days. I received it for Christmas 1960.
I also still have a 1961 Christmas present. The Second Scientific American Book of Mathematical Puzzles and Diversions based on Martin Gardner’s column.
I have a birth certificate, but I don’t think it’s original. I think it dates from when I got my first passport in 1966.
A “sea bottle” that my father (a chemist) made for me when I was (I think) 9, 42 years ago. It’s a mixture of dry-cleaning fluid and water with one fluid died blue; they don’t mix, so it looks like the ocean if you tilt it and watch the fluids make waves. The blue dye has separated out pretty badly from the fluids so it doesn’t look as cool as it once did. But I love that bottle as a token of how cool my Dad is, or at the least how I looked up to him when I was young. Hell, I still do, he’s pretty cool. He and my son made gunpowder a couple of years ago!
My oldest is a 1946 radio that I used to listen to as a child. My parents got it right after the 2nd WW, and I remember sitting on the wooden floor in our house listening to it in the faint light…from the glow of the radio tubes and the lights glowing in the foot of our floor lamp. The lamp had lights above too. Quite the atmosphere for listening to The Shadow.
I restored the radio about 12 years ago, and it still works. Nice soft tones to match the soft glow of the dial.
I still have the first book I bought with my own money. I saved up my allowance, my dad matched it, and I got a Time/Life book, The World We Live In, dated 1955. I got it soon after it came out; I remember him helping me fill out the order form and making a dust jacket from a sheet of plastic. I would have been 5 or 6 at the time.
I also have the matching volumes The Wonders of Life on Earth and The Epic of Man, in the same plastic wrapper, but they’re 1960/61.
I have real Tupperware my mother gave me when I graduated high school and left home in 1981. I still use it daily.
I have my Teddy Bear that I received for my first Christmas. Teddy Anne Bear will be 45 this Christmas.
I have a small silver spoon with my initials engraved on the handle given to me just after I was born. I guess that makes it nearly 50 years old. I use it to stir my tea.
I thought it would have been one of the stuffed animals I kept at my mom’s house, but then I remembered my poster!
When I was born, my dad put a framed poster of a Ferrari Testarossa in my nursery, as that was the “it” car of the time. I think. Because I don’t know jack about cars. But that poster is still there, albeit slightly faded. And it’s exponentially cooler looking than most of the possessions I’ve bought for myself. 
The day I was born, my Dad bought me a teddy bear. I still have him. He’s pretty beat up, and now sits on a shelf. But he’s likely the thing I’ve owned the longest.
My teddy bear, because he’s my teddy bear.