I have the same problem. I am adult verified, age verified, Payment Info on File, have set preferences to include Adult content and have been a resident of SL since 2006, and yet still cannot access parcels where the land-owners have restricted access to age verified residents. However, I have been able to access everywhere in Zindra that I've tried so far. This must be a bug in the program that checks age verification, either when the verification is first done, or when it does the check at the time of access. It would be useful if we could check on our Second Life account page whether age verification has been set correctly or not.
With regard to comments about whether it's a payment info problem or avatar age, etc. the message that you receive when you try to go on land with the various access restrictions is generated within the system, and is very specific about the reason for restricting your access to the parcel.
You can check your age verification status here: http://slurl.com/secondlife/Concinna/154/38/32.
PS. I went through the age verification process again this afternoon, successfully as before, and after re-logging, went back to the above-mentioned landmark to check the message that it gave me so that I could put it in my post. Surprise, surprise, this time I had no problems! I was accepted as being age verified! I'll check again tomorrow just to make sure that my status doesn't revert back in the meantime, but hopefully it will be fine.