I have never heard of an insurance company not covering sterilization because of age, that almost seems discriminatory. Unless the patient was a minor or something. I have heard of doctors not wanting to do it on very young women, but never after 30.
I had my tubes tied when I was 27, no problems with the doctor doing it and no problems with my insurance company. I imagine if that was a common problem, my doctor would have told me to check with my insurance company beforehand, to make sure. They never questioned it at all.
The only thing about getting it done young is that the doctor may want to counsel you multiple times before doing it to make sure it is not a rash decision. (mine did) To be honest, I think that is an ethical way to handle it no matter the age. They should make sure the patient knows what she is doing and is absolutely sure that her mind is made up.
You (general you) really should go into it with the idea that it is permanent, because even with the improved rates of reversal...there is no guarantee. I think that is what doctors want the patients to realize.