DS1 is 11 and Australia where we are now is his 4th country. I can't really agree with MrsFlannel. It has nothing to do with selling the pool or the dog.
The DC have feelings and are truly sad to leave friends behind. Of course moving to a nice place is better than a shitty place, so they don't have to complain about that as well. But a lovely beach will never substitute a person.
We have move lots because of my husband's job, so I have done it several times at different age, and in my experience, the longer you wait the harder it gets. Because friendship will get stronger.
Even for the schools I wouldn't wait too long. Language is not the issue. Every country has its own way of teaching - learning, and you need to adapt and it is easier when the kids are still in the earliest years.
I was worried for this move about my eldest, 11 but the one who really badly suffered was my youngest, 8, who started soiling himself, ripping his nails, and crying himself to sleep.
But then Australia is very far from everyone and everything, and having moved several times has numbed our novelty/adventure factor.