You have no way of knowing how old people are on imgflip.
The only way to actually check is to require a Government issued ID. This would eliminate some of the kids and also 50% of the traffic.
I'm a mountain gorilla. With a cell phone. My drivers license was made on photostocks.
So you can destroy the ad traffic by requiring some kinda multi step ID verification or live with the fact that some of the teenagers might not be teenagers.