Even old birch bark should light well because it is quite reinous. As long as it is dry and you shred it fine enough you should have no trouble lighting it with a ferro rod.
I don't know how you are "shredding" the bark, but I have seen some people struggle to light birch bark because they cut it into strips rather than making it really fine. The strips can still be as thick and as hard to light as an untouched piece of bark. If you have not yet tried it, try scraping the bark with your knife blade at 90 degrees or so to the bark to produce really fine papery scrapings. these should light easily.
I find it easiest to do with a good sized piece of bark, scraping it with a really sharp knife. I scrape it to and fro along the "grain" of the bark (Imagine that the bark was still on the tree, then scrape it left to right). This makes two piles of scrapings, one at each end of the scraped area. I then scrape the two small heaps into one big heap and light it with the ferro rod. Let us know how you get on.