Allison,
The ladies are right that Charlotte Mason did not advocate starting formal grammar instruction into age 10 or even later. She felt all grammar could be absorbed in a single year with review after that. I was definitely on board with her idea when we switched to a VERY CM education for my oldest son during his third grade year. But, even CM's own grammar book (republished by Karen Andreola as Simply Grammar) needs to be used more than once over time in order to cement the grammar concepts. Catherine Levinson, a leading CM educator, mentions using Simply Grammar two or three times to get retention from her kiddos. So, grammar is not a one-shot deal as we'd love it to be.
Also, due to the upped requirements in states with writing assessments, we found it necessary to do an earlier introduction to formal grammar than CM proposed, in order to have a common language about how to write better. For example, to point out whether kiddos are writing in complete sentences, they need to understand what a subject and predicate are and what is missing from their sentence (making it a fragment).
If we wish to have the child add more detail, it is VERY helpful for kiddos to understand what adjectives and adverbs are and how they function within a sentence. When we ask for written answers, it also helps if they can compose their sentences in a way that makes sense (with parallel usage). When asking kiddos to fix sentences that aren't grammatically correct, it helps if the kiddos know their basic parts of speech.
Also, for the mechanics and usage portion of standardized tests (Iowa Basics or SAT's) kiddos need to understand use of commas, end punctuation, and capitalization. So, even though it makes sense to delay formal grammar instruction, we are forced by the state to show progress in these areas by the way we report to them. In our state of South Dakota, we also have writing assessments that must be turned in yearly, which are scored partially based on grammar.
At HOD, we use copywork, dictation, oral narration (and later written narration), poetry, and literature in a very Charlotte Mason way. We delay formal grammar instruction until "Bigger...". However, at that point we find it easier to do a little grammar instruction each day rather than waiting for a heavy introduction to grammar later. That happens to be our philosophy.
If your heart is leading you toward a different grammar program, by all means follow it! That is the beauty of HOD. But, for the record, I will say that Rod and Staff while not flashy, does get the job done. For the time I put into teaching grammar in the past (including "Intermediate Language Lessons"), I will say that Rod and Staff sticks much better making the teaching time better spent for me!
Blessings,
Carrie