This seems really important for you to notice.
In one moment, she can seem absolutely committed to a hard fact about child raising.
Give it a few weeks, and she's backpedalling on it.
She told me a while ago: "I'm not sure what to think about the age limits with lego anymore..." and implied that she may have been wrong. I didn't respond much.
Not responding seems like the best response, nice job!

Have you tracked with any of member thankful person's threads over on Bettering? She has chosen to, instead of JADE with her partner's intense assertions, acknowledge the assertions by saying "mmh hmm" and leave it there. Acknowledgement of the outburst without engagement with the content of the outburst has worked well in their situation to defuse many conflicts.
I'm thinking about suggesting that with lego we could allow our kids to be one year ahead if they seem to meet that skill level, meaning they can have 7+ when they're 6 etc. Or should I only carefully suggest that we pay attention to our sons skill levels instead and let her make her own conclusions?
That seems like more re-engagement or re-animation of a topic that might be best laid to rest.
What do you think would happen if you just let the kids play with whatever legos they wanted (within reason), and didn't re-open the door to that conflict with your W?
Also, do you think your W is going to bring up the topic again, now that she has "implied she may have been wrong"? If you think she might, you could consider not raising the topic yourself, but waiting to see if she brings it up again, and only then engaging a bit about it. You unearthing a topic that she might have "closed the door" on (by hinting she was wrong) might just create a conflict again.
She is probably not skilled enough to say directly "you know, babe, I've been thinking, and I was wrong about telling the kids to only play with certain age ranges of Legos. Moving forward, it's all good". If she had said that, I wonder if the conflict would feel more resolved, like there was nothing else there to talk about or figure out?
But she doesn't have that skill set, so you may need to try treating her "hint" of "I'm not sure what to think about the age limits with lego anymore..." as the best and most skilled way she has to say she was wrong.
I don't know her at all like you do, though, so you can think about if this seems to fit.
Anyway, my short version is -- gut feeling is telling me to see how things go "just moving forward" and not trying to rehash the Lego stuff with her. Just letting the kids play with whatever (again, within reason -- no choking hazards or expensive sets

) and not trying to "get agreement" or "make sure it's clear" etc.
Thoughts?