If your old articles are still relevant and valuable, then it will naturally get good traffic. However, Google gives some slight advantage to fresh posts and so it is possible that your old articles may not get the traffic it deserves.
Here are some tips to improve traffic to your old articles:
1. Review your articles carefully. Check for spelling and grammar errors. Correct all the errors. This will help in two ways: it will change the content and will make search engines think that the content is updated and it will improve the quality of your article/site.
2. Keep updating the information provided in the article. Over a period of time, some content will become outdated. You can update them periodically, which will help you improve their ranking
3. If you get lot of comments, that will keep your articles fresh in search engines. So, encourage others to comment on your articles.
4. Respond to all queries posted as comments for your articles.
5. Keep deleting all spam.
6. Keep linking all your articles in the blog. You can provide links from old articles to newer articles and also provide links from new articles to old articles.
These tips will certainly help you improve traffic to your old posts.