Great question - The ages recommended are not set in stone - they are just recommendations. Each family has different goals for homeschooling, and it's just as important to choose placement with those goals in mind as it is to choose curriculum with the placement chart in hand.
HOD's curriculum is not horizontal in skills. Each level gets increasingly more difficult, and requires dc to do more and more academically. These skills are woven into each subject area rather than all taught separately (i.e. written narrations are taught in PHFHG in a guided way along with the history, creative writing is taught alongside the poetry, etc.). So, skills intermingle, which I love because this is a more natural way to learn them (as opposed to writing a written narration about just any topic, or writing creatively with no real focus on what's being studied already, etc.). This is a huge strength in my mind - connections between learning has equaled retention for my dc.
So, you just have to know when you look at the placement chart, it shows what skills your dc need in place to do the plans as intended, and it shows the ballpark of age ranges. If a person has a 3 yo and chooses to have them do LHFHG, that person knows going into it, it could be hard for the 3 yo to do it as intended - so if it's too hard, that's not the program's fault. Likewise, if a person places a 12 yo in Bigger Hearts, that person knows going into it, it might be easier for the 12 yo - so if it's too easy, that's not the program's fault.

This just all becomes part of the overall responsibility we each have as parents to choose the best level of program for our dc - after all, we know them best!

IMO, this is one of the greatest privileges of homeschooling, and I'm so glad we have the final say in it.
Ages are just guidelines - I shudder to think if I was compared to all 38 yo's and tried to be placed in a curriculum. We'd be all over the place in skills - and dc are the same way. So, the placement chart is a guideline, the age of dc is a guideline, the goals of your family is a guideline, and your own individual dc's abilities are a guideline - whew, no wonder we take our sweet time figuring out how best to homeschool our dc!
We have moms using HOD very successfully within target ages, and we also have moms using HOD very successfully stretching the ages - so, I pray you can find how best to weigh your decision for which guide(s) to use for your situation!
In Christ,
Julie