My son is at year 7 (11ys) right now. He's the first year here in UK. Next year 20/21 will going to secondary school Year 8 and studying at day school..... Planning to have boarding when he's ready..... Probably at around year 9-10. depends on the children's character and ability...."try gradually".......
Year 7: day school (mom is staying with kids)
Year 8: day school term 3 (can let them starts 5 days boarding)
Year 9: 5 days boarding, (term 3 starts full boarding)