It's very difficult to say, because freestyle football is still at his beginning, and we could consider ourselves as the "1st generation of freestylers", so we can't really say when to stop freestyling. For example: most professionnal footballers stop playing professionnally at the age of 38-40 (?), because there have been so many footballers who have had to stop it, because they weren't fit enough anymore. In freestyle however, we're all mostly young, nobody has ever become as old that he got injured du to his age, or that he wasn't fit enough anymore to freestyle efficiently. Of course you still can juggle at the age of 60, but this hasn't much to do with freestyle anymore. We only know from Abdullah or Mr. Woo (how old are they?) that you won't need to stop freestyling at the age of 25. So lets find it out! I think when you get a job, a family and a house etc., the majority of us would stop/give up freestyling, because of a lack of time.
Personally, I'm thinkin of freestyling maximally until the age of 28-30 years.