I'm not sure if there's a lower limit.
The youngest dancers in the history of the show that I can think of would be Vivi-Anne and Mackenzie in the first season, who I believe would have both been 6 at the time. There may have been a change in child labor laws since then, but I'd guess 6 would still be theoretically possible (and maybe younger.)
Most recently the producers have been looking for dancers 13 and under, but in the past they have had dancers probably about as old as 17 or 18 (Kevin Cosculluela, Nina Linhart, likely some others that were 17+. And I think Payton will appear in Season 5, when she's 17.)
And of course there are moms, Abby, etc.