By using conditions in the Mailchimp action after submit, you can let the form execute the Mailchimp action only when the specified conditions are satisfied.
You can set the Mailchimp action to execute only when the customer age input value set in your respective age number field is more than or equal to 18.
You can ask a question and get an answer via a select field to make a decision whether to add them to your email list (via Mailchimp) or Google sheet (via webhook, where you can set the opposite condition).
Furthermore, you can specify the condition(s) in an expression. You can specify multiple expressions (one condition per line), where they are and-connected, which means the action will be executed only when all the conditions are true/satisfied.