Smelly Clark wants to change his age from 16 to 21 so that he can become a barber.
Smelly is disappointed that all he can currently do around the shop are menial tasks, such as sweep up. If he could legally change his age - like people change their names - he could become a barber.
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___________________I'm guessing that Smelly failed to legally change his age since instead of acting grown up in future episodes, he remains a 15 to 16 year old kid.
Smelly's ambition to be a barber would later lead him to butchering Rush's hair. Perhaps it's a good thing he didn't get his age changed.