I don't think "grownup" is a bad description either. One of the fellows in the masters swim club here showed up about 5 weeks into the start of the sessions after not swimming at all for months, kept glaring at me because he couldn't pass me, and insisted that he was "getting too old for this". Apparently he doesn't do any swimming on his own at all in the off season, only the club practices between September and June.
This points to a certain mindset, of having to be told what to do, when to do it, and relying on external motivation to even get to the pool. Fine for an eight year old, not so good for an adult.