According to the The Walking Dead wiki Carl was 12 years old when the show started, although the actor was 10-11 at the time. How old is Carl when season 8 started? He has a visible beard by then and got about as tall as Rick.
In the real world, he aged 8 years, whereas in TV world I believe based on Kirkman's comments no more than 5 years which if he started out around 11 would make him 16 at most? Lots of inconsistencies in his and Judith's aging but puberty in a post-apocalyptic world must be tough...all that stress ages you quicker I guess while Judith as a baby is still protected and shielded.