The issue of being unable to delete old posts or pages appears to have been getting better and few are still having problems with it, so hopefully you’re not one of them. Nevertheless, if you have problems, cross your fingers because this problem can be difficult to narrow down.
It tends to be, most commonly, that these problems stem from bad plugins or themes. If you have a problem being able to delete posts or pages on your site, start by deactivating all of the plugins. Afterwards, if you still have problems, switch your theme to the twentyten theme. See: Change WordPress Theme in PHPMyAdmin
If these changes fix it, then turn your plugins back on and deactivate them one by one until you find the plugin(s) or theme which is causing the problem.