Yes, mostly the cut off is 65, from what I understand if you get in before, like 64, then you can carry on and stay on the same policy, but they don't sign you on new if over 65 - as mentioned there are some with different rules of course....
The most important thing is to go to your local clinic/hospital near you that you would be using and check if they accept the company you want to sign on. I remember sometime ago CMP refused some big names due to some dispute of some sort, imagine, that would be bumber if you live in this area....!