Ryelands will fatten easily on grass alone and on good pasture will soon get overfat.
If you are weaning them June, then I presume they were born Feb/March. You say they are growing well, so provided you keep them on good grass and worm them regularly (unless on completely clean/ungrazed by sheep pasture), then they should be ready July/August.
However, as previously mentioned, you go on condition rather than age. So you need to get someone to show you how to tell when a sheep's fat. As a rough guide - catch a lamb now and feel the vertebrae at the top of it's tail, just where it's attached to the main body. You should be able to easily feel the gap between each vertebra.
As the sheep fattens this gap fills up and if you cannot distinguish the individual vertebra by feel then the sheep is too fat.
You are aiming at the point where you can still feel the individual vertebrae, but the gap between is almost filled up.