No, RevenueWell will not send individual direct mail communications to parents if a child under 18 was supposed to receive something. In other words, patients under 18 may get skipped for certain communications. For instance, if you use direct mail for Recalls, a patient under 18 would not receive a recall notice.
If you'd like to set up a minimum age requirement for direct mail, click on "Direct Mail Settings" in the Settings tab. Note this setting only applies to automatic communications, not custom campaigns.