Component :

<aura:component>    
    <div class="slds-m-around--xx-large">
        <div class="slds-form--stacked">
            <div class="slds-form-element">  
                <div class="slds-form-element__control">
                    <ui:inputNumber aura:id="inputAge" label="Enter your age" class="slds-input"/>
                </div>
            </div>
            <div class="slds-m-around--medium">
                <button class="slds-button slds-button--brand" onclick="{!c.validateAge}">Validate Age</button>
            </div>
        </div>
    </div>  
</aura:component>

Component JS Controller :

({
    validateAge : function(component, event, helper) {
        var inputAge = component.find('inputAge');
        var ageValue = inputAge.get('v.value');
        if(parseInt(ageValue) < 18){
            inputAge.set("v.errors", [{message:"Age should be more than 18...!!"}]);
            return;
        }
    }
})

Output :