This is a good question, as different curriculums certainly take different approaches!

It can be somewhat confusing, I know! But, take heart!

HOD has a well thought-out, clear answer to this question. In fact, HOD has spent quite some time pondering this question prior to the writing of the guides, which is so helpful - as the guides then were written with the decided upon approach in mind from the very start.

This makes the approach very successful.
First off, HOD keeps the suggested age ranges for using guides to smaller increments. This keeps the range of ages to a manageable amount... it's a sort of checks and balances for dc to be placed in guides they can truly each successfully do. Extensions in the area of history are then added, and ample levels of LA reading, writing, grammar, and math are provided, so older dc can be 'extended' quite effortlessly in these areas as well. Even in the case where more LA/math is needed, simply getting the next guide and utilizing its right LA/math side of the plans works quite effortlessly too.

However, I do want to say that HOD does not just standardly say 'teach to the youngest' or 'teach to the oldest.' Instead, HOD says, 'use the placement chart as a guideline to determine if dc should be combined or if they should be separate.' If the placement chart shows dc can be successfully combined, then guide selection is based upon the younger of the two guides in question, as beefing up is already planned in the guides. Ironically, sometimes the 'youngest' places higher than the 'oldest' - so placement is really determined more upon individual skills of each student. And then, after all that, the final consideration for placement is based on other things as well... size of family, outside influencing factors (i.e. mother working, father traveling or deployed, finances, parents' health, children's health, mother's health, number of activities, etc.).

It's no wonder we homeschool moms spend so long pondering placement!
Anyway, Carrie has written some excellent posts in regard to this topic. Here are some links that will help...
viewtopic.php?f=6&t=9338
viewtopic.php?f=6&t=9339
viewtopic.php?f=6&t=9341
viewtopic.php?f=6&t=9342
viewtopic.php?f=6&t=9343
viewtopic.php?f=6&t=7019
HTH! Happy reading!
In Christ,
Julie