You do not have to acknowledge the existence of anyone under the age of 20. Except at your own discretion, otherwise family gatherings could get quite awkward. Or if you have created some under-20s of your own. Choose another age threshold as you see fit, 20 seems to be about right for my sanity.
This saves you from having to deal with the vast majority of the barely sentient bipeds that parade themselves around our towns and cities.
You do have to watch where you’re going though – take it a bit too literally and you may find yourself one day sat in casualty, totally unable to explain how you got a sharpened baseball cap embedded in your spleen.