Do you just naturally start not to think about it?
I feel like it slips up high again out of habit, not because it hasn't dropped a fair bit. When I get caught up in conversation, I squeak constantly and it gets super low. But then it doesn't seem to stick around all the time...not because it isn't there, almost like because I can't find it again.
I don't think that you naturally start to "not think about it" - especially for older guys. Most of us have spoken with a "girly voice" for 10+ to even 20+ years, so it can sometimes be a constant thing to try and remember to not talk in the same way that we once did - at least that's my case.
I can still make my voice sound somewhat female if I try, but I think that it's dropped to the point that even trying to sound girly just sounds like a young male's voice. I am pretty bad at still using that "high" tone when I'm on the phone, so it's an effort in that I have to deliberately use my lower pitch now when I speak to people on the phone. So for me anyway, it's still an effort not to try to use my voice in the same way that I once did.