I disagree with the majority of people on this post. Being a 16 year old that races both karts and cars I feel that the senior age is about right, but maybe should have more leniency for younger drivers to move up. If you look at the top drivers in the past few years in TaG and Rotax, very few are older than 18. The two team USA members so far this year are 16, the USPKS champs have been young and even stock moto is much of the same, bar the Factory drivers.
Karting is the stepping stone into the rest of motorsports so it is ridiculous to prevent the drivers who have aspirations to reach top forms of car racing from driving at the senior level. The ages of drivers making it to NASCAR, F1 and Indycar are being lowered significantly, so by keeping a kid in junior you are holding him back from learning more in tougher fields that will prepare him for higher level racing. It would also be a confidence killer for the kids who move up to junior at 12/13 who have to race against guys like Askew and Craig (assuming the age is say 17/18), they would stand no chance. I see the class ladder as a progression where you can find success all through your time karting. The difference between a 13 and 18 is a lot bigger than 18 and 25 maturity wise IMO.