"only young teachers?"

No.  I live in small town outside a 3rd tier city.  I'm not a young teacher.  I'm also not old, and not a teacher

If you head to the bars or clubs in the 2nd tier city or 3rd the main people you will see are the younger types. But I have met factory managers in these cities who have brought there family across for the ride. 

Some young people think it is a better experience to head to the smaller cities than the big ones.

I'm a 48 year old married man with a 2 year old daughter teaching in Nanning, a 3rd tier city.

Why? Nanning is my wife's home town, and teaching is my second career which I discovered as a vocation after my first career came to an end. We have no intention of putting our daughter into an international school but will put her in the best Chinese school and examine our options once she reaches High School age.

I like Nanning it is small enough to be friendly and not as wild as some of the 1st tier cities allegedly are, and of course we have the support of the family.

  Yep, i've been living exclusively in 2nd tier cities for a number of years and i've known only a few families. All have been supported by teachers though, including my own. For the most part, as you suggested, foreigners i've come across have tended to be youngish travelers, and yes, most have been teachers.

I lived in Guilin and being a man in my 40's I was definitely in the middle. Half the foreigner I knew were under 30 and the other half were over 50.  Knew a scattering of 30+  year olds, and only 1 or 2 forty year olds.

If you are in Nanning, you will find a bigger variety.

I lived in Dalian for a while (with a paltry population of 5M) and there were lots of families there that were older and not teaching.  They worked for a lot of the high tech companies (IBM, HP, Cisco, etc...) or the ship building industries.  Either way, there was a rich mix of expats of all ages and job descriptions.

On the other hand, now I am living in Wuhan (the armpit of China) and there are very few families in the area I live in.  Most of the foreigner families here are French and they work in the Automotive industry.

I live in (as I suppose) 3rd tier, or even less (Kaiping).

I'm not young (37), and not a teacher. I work for european company, that makes some production in this region.

Single, but decided to live in China. Where exactly... it is another question.
Kaiping is relatively small city, air is quite clean, people friendly, what else is needed for me?
OK, if there will be a "family case" I may decide to change point of view.

You have the occasional representative or factory manager, but that's about it.

Naw. I'm not young, but I am a teacher. I live in a 3rd-tier metropolis. Quite satisfied with it. There are a lot of different ages here. The young guns are teachers and students, while the older ones tend to be teachers and professional managers.

Most of the younger ones hang out around the university sector across the river, so I don't get to see them that often, as I'm more downtown. Here, the demographics do shift slightly, as it's closer to the businesses.

I am IT professional and I am also living in 2nd Tier city..

In Xi'an, it's mostly teachers. You'll find a couple of VPs for Aerospace and Hotels.

All cities have old teachers as well as young.

Old and young engineers.

Businessmen.

Bars and clubs always have old men as well as young.

I lived in a 3rd tier city, and I was the youngest female expat, as well as the youngest ESL teacher in the city. There were 2 other middle aged male ESL teachers at the other college, and an older woman who taught English to doctors at the main hospital. There was also a couple originally from South/Central America (I'm sorry, I can't remember which country) who had lived in the US for a while, then decided to move to China and had a baby. I'm not sure what she did, but he was teaching English and Spanish at a middle school. There were also a few young Indian guys interning at the hospital. Those were the only expats I knew of in the city.