Agree that your problem seems to be much more injury-related, rather than to do with age.
Get the achilles sorted and there's no reason why you can't run and race for many, many more years yet.
If you're relatively new to training seriously, you can also still look to improve and set new PB's. Admittedly, if you were racing in your '30's and '40's (let alone earlier), declining race times are a fact of life, but nothing to get hung up about.
Beyond 50, cross-training and stretching become more important, as the one thing you want to avoid is injury and I believe they do help a lot.
DJ (53)