I've never heard of a rule-of-thumb for this. I think if you are unhappy with your job / career - you should definitely change. I think in some ways at an older age, you are more prepared for a career change since you've had lots of time to consider what you like / don't like in a career and you likely have some money saved up to take time off of work to get prepared. If circumstances in your life present the opportunity (like a lay off or something) then I think it should be "carpe diem"

In terms of career change there's no age limit to follow as long as you can still handle things right everything won't be too hard for you

The career is like a Mall.
You have to grow always.In case you jump and rich faster to a new level without having the proper knowledge then you'll be out of the Mall for a while.The knowledge is credit.
You finish your credit ..then you connote just get further.
The age in career development must have been very important in the past but now,... you can do it easily .

I know people that have work in the same Company for more than 20 years and ..
What happened next?
If the company just closed the doors ..lost and depressed the fellow have to move on..

In this way a career shapes and wisely the next step can get you closer to the top ...without thinking.

To change then the career will never be questionable.

Why?
Humans are learning continuously.

So, in this way if you're forced to do that or ...you just want it ...nothing should stop you.

Many people without diplomas or coursess have achived a lot in their lives.
I do hope that the age will never be an obstacle to make a career change.

I do personally agree that you should stick to one single company but,still,you should consider a new challenge sometimes.
To all that need to make a change in their life or just simply changing the jobs( career) all the best.
Good luck