I have just returned from taking our first 5 cades for slaughter this year, they were fed creep up until about 12-14wks old and then out on good grass, they have grown very well, the lambs we took today were 5 1/2 months old and could have gone a few weeks ago but we were waiting for the wormer withdrawal to end.
If your grass isn't great I would probably keep feeding but if you've got enough grass to sustain them I'd get them off it and just feed a token amount to keep them bucket tame which is always useful.