can i only use meta tags and description when starting my blog post, or can i instal meta tage?

[Mark]

even if the blog's posts are a few months old? is it too late?

[Tommy]

Hi,

Depending on what Content Management System you are using, you might be able to edit it anytime. In most cases, you should be able to edit your article and update the meta description and keywords.

Will need more information to answer but I am sure that if you are using Wordpress and using All in One SEO or Yoast, you should be able to edit old blog posts.