DF = data.frame(
    group=c( rep( 'A', 6 ), rep( 'B', 6 ), rep( 'C', 6 ), rep( 'D', 5 ) ),
    age=c( 9.00,  9.50,  9.75, 10.00, 13.00,  9.50,
          11.00, 10.00, 10.00, 11.75, 10.50, 15.00,
          11.50, 12.00,  9.00, 11.50, 13.25, 13.00,
          13.25, 11.50, 12.00, 13.50, 11.50
          )
)