I have just started using the 5-7 curriculum for my 6 year olds and they seem to really like it. My biggest challenge has been feeling like I need to memorize the instructions each day and then teach the concepts with the exact terminology used in the directions. The teaching process doesn't seem flexible yet for me, maybe I'm going about teaching MBTP in too restrictive of a format.
As far as age appropriateness, my kids are in their K year and I like how this curriculum has multiple options to adjust for ability level with different levels of projects depending on how much reading and writing my kids have achieved.