Sounds like it's not talking to the field.

Do you mean form/subform? If so you need to change the wording of the calculation a little to talk to the subform.
Forms!FormName!SubformName.Form!FieldName

Or do you mean another form entirely? Is this other form open when you open the form with the calculation?
Forms!FormName!FieldName

If it's not open, you'll have to get the value from the table, probably with a DLookup

so i'm trying to fetch another field from another (parent, in one to many relations) table (and even form based on it) for creation of expression. Aren't they supposed to talk to each other when they are in relationship (in this case direct)?

They are related but may not be being told to communicate how you want it to with your code. So if it's in another table with an established relationship, just use a DLookup

Set your text field control source to something similar to this, you'll have to change your field/tablenames into it

=DLOOKUP("DateOfBirthFieldName","ParentTableName","RelationIDField=" & RelationIDFieldOnForm)

That will provide you with the date of birth, and then you can calculate the age with it.

This video may help you determine your issue.

Form notation  {https://599cd.com/blog/display-article.asp?ID=1647}

Value From a Form

Are you actually using a form / subform combination, or are you trying to read a value from a table directly? If the latter, use DLOOKUP.