I'm 50 and would like a career change. I've been considering ATC training at NATS. Again, am I too old ?

They would probably find a way not to have you! Have you been into a busy ATC unit to see what is involved? And there is also the medical to consider at your age.

As HD says, NATS would be unlikely to accept you for their rigorous training regime, however if you have the money, why not self finance through an ATC college like the one at Gloucester? Alternatively, you could study for and take the exams for a Flight Information Service Officer and get a part time job to see if you have the aptitude. You'll need to obtain a Radio Operators Certificate of Competence to start training if you take this route.
Whatever you choose, I strongly recommend you visit both and ATC and a FISO unit to see what is involved.

I attended NATS college at 42 as a private sponsored student and got through. Went on to complete a procedural and radar course and am now an OJTI and an assessor. Any thing is possible if you really want it.

From what I have seen over here in the sandpit - the learning curve for the average person starting out tends to get very steep once they hit the mid-30s mark.
We find that where the vast majority of the older folks fall down is in the application of the knowledge gleaned during the theory parts of the course.
A lot who do really well in the written exams go to pieces when they get to the sim.
(Then comes the somewhat arduous process of trying to explain that to a student who thought they were world-class with their 95% in the written.)

Try to arrange a visit to a unit and if you like what you see, look at doing some kind of assessment test like "Sure Select".
Dreams are great, but if the tests tell you that ATC is probably not the wisest of career choices for you, they are usually correct.

That being said….Good luck!

Farrell