The realisation that you now understand what people mean about younguns' music sounding like "it's not music it's just noise" - mainly for what they play on mobiles on the bus.
Realising you are old enough to be a colleague's mother. Similarly colleagues going "who" when they have no idea who/what you are talking about.
Realising the number one song on the day you were born is not an old fashioned cult classic.
It not being an option taking a coat and flats to get home in on a night out.
Choosing comfortable, more sensible shoes (ditto for underwear and nightwear).
Walking through BHS/Wallis/M&S and seeing things I'd actually buy.
Developing an interest in watching/reading the news.
Avoiding the "student" buses and kicking out time when the schools finish.
Oh, and one I saw on FB, when your friends start having children on purpose.