Without your files to test against, we can't do much at all - so start by "breaking down" the Linq into separate statements:
var files = new DirectoryInfo(fileLocation.Text).GetFiles("*", SearchOption.AllDirectories);
var selected = files.Select(p => new FileInfo(Convert.ToString(p))).ToList();
DateTime limit = DateTime.Now.Subtract(TimeSpan.FromDays(Convert.ToInt32(fileAge.Text)));
var where = selected.Where(p => p.CreationTime < limit).ToList();
var where2 = where.Where(p => string.Compare(p.Extension, extensions, true) == 0).ToList();
Then you can step through looking at each stage in the process to see exactly what is being processed and what results you get.
Hopefully, that will give you some info on where teh problem is.