When I find myself thinking of things that seem "new" to me (films, music, tv shows) and they're actually 20 years old or not far off! Seriously anything post 2000 seems "new" to me
Looking in the mirror and seeing not my mum...my gran!!
I've been dressing for comfort for years now.
When you can no longer be arsed to make the effort to learn certain new things - snapchat, tik Tok... I'm so not interested Facebook & Twitter are my limit
Not "recognising" current pics of old friends on fb (ones you don't see often, I was an army brat so many of mine are scattered worldwide) because in your head they're still the age they were when you last saw them or were spending a lot of time with them. I've had friends make similar comments re how I look now too.
When friends are becoming grandparents! We can't possibly be old enough - then you do the maths and sadly we are!
I like asking youngsters nearly everyone in my office if they know what the USSR was. My favourite answer Is it in the USA? I still find myself reminding myself that Czechoslovakia and Yugoslavia no longer exist! I find all that area of geopolitics very confusing
Realising that I am significantly older than 34 countries, and comfortably older than the internet. oh fuck! How'd you check that? (And how old are you? I'm hoping older than me but doubt it somehow)
Reading posts on mn by posters who are clearly mothers, even to teens but who were obviously born after I left school so in my head they're "just kids" - then - again - doing the maths and having to accept they're in their 20's and 30's and a perfectly normal age to be mothers.
@BillywilliamV I'm amazed you weren't lynched!
I had a conversation with a call centre worker recently which made my heart sink when he actually said "yes I grew up with iPhones so that's what I'm used to I've never tried an Android" - I got my first mobile phone in my 30's!!
Totally agree re Gilmore girls! Lorelai basically threw a 14 year tantrum! Because not unreasonably her parents weren't best pleased she got pregnant at what? 16?
The episode/scene that breaks my heart is when Emily sees the HUT she and Rory were living in when Rory was tiny and says something like "my god she really must have hated us!" When all they really wanted was for what was best for lorelai and Rory in the circumstances.
Re the 80's "not being history" while I sympathise consider:
1989 was 31 years ago.
When I was in school ww2 was being taught as history this would have been from 1983-1988 ww2 was 38 years before that and I well remember my parents and certainly my grandparents making the same argument!
@MsAwesomeDragon I have friends my age that are teachers, they started to feel old when they found themselves teaching the children of our peers - they're now teaching the grandchildren and feel VERY old!
Arguments when a sibling taped over your favourite show with theirs!! Looking at you bro constantly taping bloody sport over quantum leap!! 
getting blank tapes for Christmas as a stocking filler and the first thing you did with them was put your name on to try and prevent the former (didn't work as little brothers ignore such etiquette - unless you're taping over his tape!)