Agree that commercial Texel types may be sent straight from weaning - but will probably have had cake to achieve good grades at that stage. Depending on the grass they're on, of course - our grass here on the Cumbrian uplands needs a bit of help for fast finishing at good grades.
I haven't done that specific cross, but I've had various Shetland x commercial crosses. For the mart, 7-9 months was enough, but assuming you'd like them larger for home use, 12 months perhaps. These figures for sheep on the Cumbrian uplands, of course.