Ok Mo-- I am about as young as the board members come while still being an adult-- I can't think of anyone else who is a regular poster that is younger than me, but over 18.
So, I will give this a go.
My oldest item is a pair of boots which are 8 years old. They are close to being ready to retire simply because I wear them often and they weren't expensive or quality enough to bother to resole. I have a couple of clothing items that are 6-7 years old, and some more shoes and boots from that era (less worn). The clothing is either something that wasn't worn often (for whatever reason, including "formal wear") or is at the rag bag stage. I also have quite a few clothes that are 4-5 years old. I then have most of my jeans and knit tops, which are 2.5.
I consider the 2.5 year-old stuff old only because I have been wearing the same 14 (give or take a few, esp. with some seasonal differences) knit tops and tees with a handful of wovens and layers thrown in and the same 3-4 pair of jeans for the last 3 years, give or take. All of them are at the point where they are showing wear, most should be tossed.
I also have a few 3-year-old blouses and dresses that are "old" because they look dated. They were trendy at the time (not-so-classic ruffled blouses, for example). On the other hand, I have 3-year-old dresses that are my current go-tos and ones I've done slight alterations on to update them (turned some flutter sleeve dresses into sleeveless). My nicer dresses are 5 and 6 and still look fresh and haven't dated.
I also have a few skirts and tops from 2003-2004 that are still in good condition, but they are dated.
I also have some things I consider old because they are 3-7 years old and I have worn them to the same type of function time and time again with the same people (I have one blouse that goes to a lot of semi-formal family and community functions, and after 7 years it is still a nice style and in ok shape, but I just can't stand to see another picture of myself in it-- if my hair hadn't changed you wouldn't know the pics span 2003-2010).
I do think that being younger does impact the age of clothes in one's closet, as well as what you consider "old", but not in the way you are thinking Mo. In your twenties, having clothing for 5-10 years generally means that the pieces are from your high school or teenage days (we won't even get into college clothes). Most women do finish developing during that period and even though they may stay the same size, the clothes fit differently. Even with no body changes, most clothing from the 14-21 era tends to be disposable and casual or trendy.
The other thing that impacts "old" is the size of the wardrobe, as others have mentioned. Since I wasn't buying premium denim and wool blazers at age 17-- I was buying 30-50 dollar jeans, cotton knit tops, and knit jackets and wearing them into the ground, the only clothing that survived me having a small wardrobe were pieces that were orphans or failures in some way or were special occasion clothes or seasonal items.
Most of my clothing that doesn't have a fatal flaw early on (stretching, piling, weird cut) lasts two years without me questioning the "oldness". At that point, it usually becomes obvious if there is too much wear or if it has dated (it happens). If it is dated or worn, then it retires. After that point in time, I am more careful about weeding and making sure things still look current, fit well, and aren't shabby. Most things don't make it past 5 years. Even my black slacks at 5 years fit slightly off, are worn, and have dated slightly -- even though you can still buy the style and I haven't changed sizes, the company recently tweaked the cut and details. For me, things that get past the 5-7 year mark have to be excellently cut for my body with very simple lines and good fabric, or "classic" items that haven't changed in the past 30-50 years like Frye Campus boots, docs, felt cloches, ect.
There are always golden oldies that are special enough to put away or just stand the test of time and you can keep wearing (a coat, suit jacket, pencil skirt, ect). However, I do think that many items visibly date in the 3-5 year area. I also think in general the 20-something population of peers is more likely to notice something slightly dated.
I see some women wearing things that were theirs in the 70s, 80s, even parts of the 90s combining them with new pieces in a fresh and funky way. However, I also see women who are wearing clothing from 5-15 years ago that don't recognize that subtle things are dating the pieces-- specifically proportions or age-appropriateness.
I too have felt dated and awkward in pieces that were seldom worn -- and I find it is just better to let them go.
