Every time I move, I have to put as much as I can in two suitcases and a backpack, so there are some hard choices to make. But I have my birth certificate and a variety of old photos among the stuff.
I’m pretty sure I still have a commemorative coin my uncle gave me in 1962 or 63. Failing that, I still have my 7th grade baseball glove.
You mean it hasn’t melted after all these years? 
I wouldn’t know where to start. I inherited almost all of my mother’s antiques and other possessions including my great whatever’s grandmother’s rocking chair which crossed the plains in a covered wagon, and Havilland dishware that’s pushing 200 years.
Hell, my bedroom furniture is pre-Civil War - and it isn’t even family.
Other than a couple of books, the oldest single thing I own is my bong – it’s about 35 years old.
I have this book too! Isn’t it lovely?
I’ve moved 5 times in the past year and a half, and I moved cross country in a Toyota Yaris a couple months ago. I’ve worn out or gotten rid of almost everything I owned 5 or 10 years ago, but I have a couple keepsakes - an antique end table my parents had for my entire life and I took when I moved out of their house, my grandfather’s shaving tray from the 1940s, and a lamp, some books, and a small stuffed animal I’ve had since I was an infant.
An enamel purse from the 1920s, given to me by my paternal grandmother when I was around 14 or so. (I’m 54). A sterling silver unicorn pendant given to me for a HS graduation gift by my then stepsister. A single gold nugget earring (I lost the other one) given to me by my dad when I was 16. A Lady Liberty Silver dollar given to me by my then husband’s best friend when our daughter (now 32) was born.
Sorry, that was more than “the thing”. But they’re all sentimental value and have been around for a long long time.
A lot of stuffed animals, hey, particularly teddy bears. I hadn’t thought many folks would list them as their longest held possession.
I still have and use the night table I had as a kid. I don’t recall ever not having it, so it is well over 55 years old. I have all the sf books I bought from the Scholastic Book Club in 6th grade,. 1961.
My wife has all the Golden books she was read when she was little, and we have a lot of the furniture that was in her house from before she was born, if that counts.
Less shame when I’m a faceless nonny on the Internet. 
I have a bracelet which was bought for me in Paris for my 2nd birthday, so 33 years ago.
The oldest thing I have in my possession is my Great Grandfather’s double barrel shotgun from ca.1910. I do keep it in my Dad’s gun safe but it is mine, it also works just fine.
There is a problem with it, it’s provenance. My GG father was a know bender of the truth(liar) and he claimed it was his while he was a guard at Leavenworth. IOW it could be much older, it looks to be modified from percussion caps. There is no proof he ever worked in a prison.
Capt
A miniature (2") cap gun and holster that was pinned on my diaper as a baby. Actually, the original diaper pin is still there, too. The diaper itself is long gone.
And it’s been yours all that time? And I thought I was an old guy.
My birth announcements. I have one that would be the kind that the parents would send out to all their friends and another very plain one from the hospital I was born in saying who the parents and obstetrictian are.
Just off the top pf my head, I think it would be my dresser. I’ve had it since I was five years old. I’d like a new one, but don’t care to go shopping.
I have some family furniture that I have been given over the years, but I don’t think that counts as much as the three stuffed animals that I treasure. Eeyore was made for me by my Mom before I was school age and I remember her sewing him for me and getting his ears not quite right. I have the pony my father brought back from a business trip when I was seven or eight and a stuffed mouse in a blue velvet jacket that may have been my Mom’s when she was little. They’re all a little battered, have missing eyes and worn fur, but I love them as much as ever. They have all followed me through every move and all hang out together in my room. I’d probably grab them first in a fire.
I still have my collection of Matchbox cars (but none of the boxes they came in). I got my first one in 1966 when I was 10. They’re in pretty good shape overall. I keep them in a collector’s case just like this one, also from the 60s. One of them is the Ford GT pictured on the case (“Matchbox Series No. 41”).
I have a few old tems from early childhood but I don’t treasure them. I just saved them beacuse they were passed down. I don’t even keep photos. I have one small phot album my exwife put together for me years after our divorce. I do have one item I treasure from when i was about 21 years old. A small box with a hinged lid for mens jewelry watches ect, it has a carving of an all white English Setter on the lid that looks just like my favorite bird dog. After more than 40 years I still look at it daily with fond memory. It sits right on my computer desk.