If you're honest with yourself and unit training officers and colleagues are honest with you, you'll know when you've had enough.
That said, it entirely depends on the environment you are working in and what pressures, on and off the job, there are upon you.
Ive seen some very good atcos do their job with the utmost competence well into their late 50s and I've seen some who just cant cope beyond the age of 45. But equally and most sadly, I've seen two "burn out" before they were 30.
As you get older, usually, but not always, as the unit becomes busier the harder it is to adapt to new scenarios - conversely the more experience you have, the easier it is to cope with the unexpected situations which may happen at a moments notice.
I can't see how any amount of research would give a precise answer to the question, experience and age works both ways.