This was/is my outcome in a similar situation, east coast family as well.
mid-season last year I moved my then 11 year old son from junior boots/skis to adult boots/skis (below 80 flex) because:
1) he was wearing size 8 US shoe
2) skis semi-aggressively to aggressive, including trees
3) when he was skiing aggressively he looked less stable from a distance on the junior boots/skis (in 140cm)
So put him in an adult boot/ski. 2012 Volkl RTM 75 in 159cm (not the IS version). Back then he was 5'0" tall, 100 lbs. It was a lot ski for him, he was skiing black diamonds comfortably but got very tired quickly (after 2 hours of skiing).
Fast forward to this year, now age 12, his feet grew: 9 US, height grew: 5''1", 105 lbs. And he has no issues at all with the ski equipment, in fact he loves them, skied 5+ hours on them without any issues or fatigue. Skiing double-blacks this year and I can't keep up with him, he passes me every time despite my best effort trying to keep up.
Part of the answer is it depends on the child and how aggressive they are. If they are leisure skiing I would be hesitant to do what I did, but my son does jumps (loves being airborne) and trees. Typically you want shorter skis for trees but the wider width helps him through the ungroomed snow. He has no issue flexing the skis.
Now I'm faced with a different issue, from January of this year till now (March) he grew a full inch...if you can believe it. I have retired and set aside my 2015 Volkl RTM 81 skis in 171cm (has metal and full rocker ski, effective edge is 168cm as result) for him when he outgrows the RTM 75 at some point. Because the RTM 81 has a full sheet of spring steel in it, at what weight can I have him use them? Thinking at minimum 115 lbs and 5'5" tall? The binding DIN settings are not an issue, he is on a DIN 4 today and the RTM 81 start at DIN 4. The settings work, saw the RTM 75 eject more than once on him properly when he was horsing around on the slopes doing some jump and landing incorrectly.
He skies about 18 times a season, the 2012 Volkl RTM 75 have about 28 skiing days on them from him.