I don’t think you’ll find a single dictionary or other reference work that defines “middle age” as including the early 30s. Wikipedia lists sources placing the start of middle age from 35 to 45, and the end anywhere from 54 to 65. It also describes middle age as being “around the third quarter of the average life span of human beings.”
Random House and American Heritage dictionaries define “middle age” as 45-65 and 40-60 respectively.
Merriam-Webster says 45-65.
One reason why I think of 30 as the beginning of middle age is that I recall a slogan of my youth: Never Trust Anyone Over 30. The implication was that 30 was the dividing line between the hip young folks and the uptight (and therefore untrustworthy) old folks.
I think there is a gap between young (under 30) and middle-aged. People in there 30s are not usually quite as into the latest trends and fads the way college kids and their peers are, but that doesn’t mean we’re stodgy old fogeys. Now that I’m 30, I admit I’m no longer young and hip, but I don’t expect to be middle-aged for at least ten more years. I’m just an adult.
Child: 0 - 12
Teen: 13 - 19
Young: 20 - 40
Adult: 40 - 60
Older Adult: 60 - 80
Old: 80+
If I had to use middle aged I’d probably say 45-60.
I’m 23.
There’s a dicionary that defines “Middle Age” as ending at 54? That means that in three years I’d be “Old”.
I’d throw that dictionary straight out!
Old always is and always will be about 20 years older than me.
Has anyone else noticed that our parents tend to be roughly 20 years older than we are?
Everyone knows their parents are ALWAYS old, no matter what their age.
Old officially happened yesterday. I helped my drywaller stock 40 sheets of drywall in my house.
Half of it went to the second floor.
10 sheets of it where 5/8"
They where all 12 footers.
I live at 11,200.
You’re old when you talk more about the past than about the present and future.
Actually, that’s the US Census, according to Wikipedia. What are you going to do?
Blame the Baby Boomers! 
You’re officially old the first time you yell at a kid to get off your lawn.
So the secret to eternal youth, then… apartment living?