There is no time limit. If you are about sixty you must have been in Nam around 71-72? You can file a claim for PTSD at anytime. All you need to file a claim is fill out VA Form 21-4138. I tried to post the link for the form but the system wouldn't allow it.
If you contact me I can email you a pdf of the 170 page book from Vietnam Door Gunners Association that explains everything you ever wanted to know about how to file a claim for 100% (or less) disability for PTSD. The book is current and up to date with examples of letters, completed forms, links to all the forms you need, just about everything you could ever ask for when it comes to filing a claim for PTSD. You can contact me via email steve-hill at unitedfilmways dot com