Overhead of using a Docker image created in a different Linux release

As Linux and Docker beginner want to simply my testing using pre-made Docker images of my target Python frameworks.
Having installed Ubuntu 18.04 LTS I discovered some of these images are made using the previous release 16.04.

Wondering if there is any overhead in using these images, as I understand the advantage of app virtualization is that leverage the existing OS resources, but in this case the Linux kernel need to have different releases.