Gravity Forms - Conditional merge tags - MULTIPLE Values in a single tag

I am using the Conditional Merge Tags shortcode in the form confirmation as found here: https://gravitywiz.com/gravity-forms-conditional-shortcode/

I've got a number field on the form where I ask people to input their age.

On the confirmation message, I'm using the merge tags to display a custom message based on the age input.

So for people who are younger than 18, I use shortcode with a message:

[gravityforms action="conditional" merge_tag="{my-field}" condition="less_than" value="18"]
You're too young.
[/gravityforms]

For people who are older than 55, I use shortcode with a message:

[gravityforms action="conditional" merge_tag="{my-field}" condition="greater_than" value="55"]
You're too old.
[/gravityforms]

Now I struggle to show the message to people who are between 18 - 55.

I'm trying to input multiple values from 18 to 55 with condition="is" like this:

[gravityforms action="conditional" merge_tag="{my-field}" condition="is" value="18, 19, 20, 21, ....... 54, 55"]
        You're the perfect age.
[/gravityforms]

But this doesn't work.

Is there any way I can use multiple values or specify something like is greater_than and less_than in the same single tag?